RGS Rear Brake Bleed

I remember a batch of Brembo repair kits for these M/Cs with incorrectly machined pistons almost 30 years ago, a sliver needed to be removed at one end to allow this particular port to open fully. Had us puzzled for quite some time.

piet
 
Mmmm the very small orifice was covered ( no pun, well sort of ) some posts back for front master cylinder/reservoir disassembly, an obstruction that disallows air bleeding of the hydraulic systems, made mention of rigid Mandolin A string for clearing of same without damage, for all you Mandolin players out there, tool you never knew was in the case. HTH j
 
50 years of motorcycles 44 years of engineering in big auto world wide and designing / manufacturing my own micro pneumatic systems and I still cant bleed bike rear brakes!
Actually I can Im good at brakes but my Cagiva Raptor with the super popular Brembo 11mm rear master cylinder had me fooled. The bike stood for 2 years I was busy on other bikes but recommissioning was replace brake fluids change oil, no matter how I tried I could not get the rear brake to work so I tried reverse flushing, then fitted an air line to the resaviour with a 5 psi on and bingo it worked. Take off the 5 psi and within 10 cycles gone again. So new seals fitted, just the same. OK complete new master cylinder....same! Then I remembered the chuck a weight on the lever or tie wrap it back "to get the air out" 5 days of that and it worked.
But wait a minute, why? Its a sealed system and not blowing my own trumpet but Im bloody good at brake bleeding. Past experience of lip seals, they rely totally on sealing sufficiently to catch initial pressure forcing them tight to the cylinder walls. Further force generates more pressure which pushes them to seal even more. Catch 22 if they stay off the walls fluid can leak past and no effective seal takes place to generate pressure to push the lip seal against the cylinder walls. My 5 psi was keeping the fluid on the pressure side of the lip sufficient to set the seal into shape and if the pressure is held on for long enough and hard enough the seal takes on a permanent optimal set.
So its not getting air out its optimising the lip seal form which on small diameter master cylinders such as motorcycle rear brakes exibit this trait the most.
I know its not a Laverda, but the principal of setting the lip seals in a master cylinder into a perfect set applies to all bikes with hydraulic brakes and goes someway to explain why motocross boys tie wrap their levers back for 24 hrs+ saying it refines the brake action, yes it will do but on small rear Brembo master cylinders it can be the difference of brakes or no brakes at all.
 

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A very common topic on here, rear brake bleeding. I have a Brembo rear m/cyl from a Yamaha R6 and I had no end of trouble getting it to pump. It exhibited all the symptoms you describe, indicating that the lips of the seals weren't making sufficient contact with the cylinder wall. In the end I think I succeeded by filling every possible cavity with fluid, basically flooding the thing. I do recall I was on the verge of tossing the m/cyl when I FINALLY got some fluid to exit the bleed nipple at the caliper ... from that point it behaved normally and has worked perfectly ever since.
 
I remember years ago a mate who was a Mechanic showed me how square-shaped brake seals had a direction to fit. I forget the details, but it had something to do with their lip shape working better if installed to suit that shape in their direction of travel. To show how long ago that was, it was around the time they started swapping steel, rusty brake pistons for alloy ones; everybody was mucking with seized brakes around then, and rebuilds were happening a lot.
 
piranha-bro2 said:
A very common topic on here, rear brake bleeding. I have a Brembo rear m/cyl from a Yamaha R6 and I had no end of trouble getting it to pump. It exhibited all the symptoms you describe, indicating that the lips of the seals weren't making sufficient contact with the cylinder wall. In the end I think I succeeded by filling every possible cavity with fluid, basically flooding the thing. I do recall I was on the verge of tossing the m/cyl when I FINALLY got some fluid to exit the bleed nipple at the caliper ... from that point it behaved normally and has worked perfectly ever since.
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Yea when you get a chance to get at least 1 good pressure maintaining cycle such that the pedal feels rock solid just once and you "set" the lip seal often systems then work from then on. I always prefill parts as i assemble them using a 25mm diameter syringe so im making fluid come out a joint as i tighten it up (just chuck water over the mess to neutralise and wipe up as brake fluid is hyrascopic but also damages paint) using thius approach Ive always managed to bleed a brake with ease but this one I could get the pedal solid and then it would just loose than again if not in 10 cycles then overnight, drove me nuts!
 
Vince said:
I remember years ago a mate who was a Mechanic showed me how square-shaped brake seals had a direction to fit. I forget the details, but it had something to do with their lip shape working better if installed to suit that shape in their direction of travel. To show how long ago that was, it was around the time they started swapping steel, rusty brake pistons for alloy ones; everybody was mucking with seized brakes around then, and rebuilds were happening a lot.
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Yes brake calipers function by deformation of the seal around the slave cylinder/s normally square in cross section the shape of the groove in the caliper body where the seal sits is designed to allow the atmospheric side on the seal to "flex" out slightly with the caliper movement. At this point the deformation force as the seal "tries to get out the gap means the hydraulic force is effectively pressing the seal to the piston walls with such force it grips the piston over the last 0.1mm movement and deforms to a romboid cross section, this deformation results in a massive elastic forces within the seal.
Once braking hydraulic pressure is removed the romboid is desperate to return to its natural square cross section and does so with a great deal of force still gripping onto the piston (but over just the deformation distance 0.1mm) and this is the distance that pulls the pads back off the disc after braking, you can be assured the pads are now clear of the disk.
Its the seal deformation that make the pads not rub and thats why you should replace caliper seals even if they look perfect, its their elasticity that goes and the pads rub.
Ive never come across any seals that are not square in cross section as you describe your mechanic mate had come across (being square means they can be inserted either way round, there is no wrong way) but it may have been a possible design route on some brakes (my understanding was always the deformation profile was withing the caliper grove that way the failure mode of seal wrong way round is eradicated) I guess the onus would be on the manufacturer to mark the seal up in such a way the correct orientation would be achieved but that does sound a bit sketchy.
In my experience calipers are designed you cant put seals in a "wrong way round"both possible ways is correct.
Over all there is a lot of "voodo" surrounding hydraulic brakes most of which is a failure to understand how the flexible bits flex and thats both intentional and a primary aspect of the design. The voodo leaves us all wondering why and how but these "tricks" to get stuff working do clearly work as for the why I think its wiper seals not set and not trapped air in my case.
I replace seals as they are an elastominer that looses elasticity over time, use correct clean new unopened brake fluid as fluid absorbs moisture when exposed to air, the clocks ticking when its opened, Prefill all parts at assembly, use water to neutralise spilt fluid, bleed thoroughly and from now on Im going to tie wrap front brake levers to the bar and for rear brakes hang 5Kg off a rear brake, do this for at least 24 hrs to "set" the elastominer wiper seal get the best brakes right away rather than experiencing a bedding in period as the wiper seal takes up that set or spending a fortune on unnecessary parts and take 3 weeks and 2 litres of fluid to sort the buggers!
 
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This bit is pure guesswork, but maybe my mate was referring to a preformed shape in the seal caused by its manufacturing process. A bit like the sharp edge formed in steel washers or clutch plates on one side as they get stamped out. He reconded he could just see a preferred side. Maybe it was just his idea.
 
Vince said:
This bit is pure guesswork, but maybe my mate was referring to a preformed shape in the seal caused by its manufacturing process. A bit like the sharp edge formed in steel washers or clutch plates on one side as they get stamped out. He reconded he could just see a preferred side. Maybe it was just his ideaMy assorted box of caliper seals all have edges that look the same to me and those in packets dont contain any instructions or markings
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As they say "Google it" and your right its a case of read the instructions, in 50 years Ive never come across directional seals in calipers but they exist:-
  • Directional Seals:
    Some caliper seals, especially those with a tapered or angled edge, are directional. They need to be installed with the wider or tapered side facing outwards, towards the open end of the cylinder, or towards the brake pad. This is because the taper helps the piston retract slightly when pressure is released.

  • Interchangeable Seals:
    Some seals, like round O-rings or square-edged seals, can be installed either way. However, even with these, ensuring a proper fit and no damage during installation is important.

  • Importance of Proper Orientation:
    Installing seals incorrectly can lead to leaks, improper braking, or even damage to the caliper.

  • Checking for Directionality:
    If unsure about the correct orientation, consult the brake caliper repair kit instructions, your vehicle's repair manual, or the manufacturer's website.



 
The explanation is great and works once the lever will produce pressure. Many on here have not got the master cylinder to produce any pressure to tie back or hang a weight from.
 
Tippie said:
The explanation is great and works once the lever will produce pressure. Many on here have not got the master cylinder to produce any pressure to tie back or hang a weight from.
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Thanks for that. Going back to my experience, I ended up being directed to this forum as thats literally what i described in a google question "why do I not get any pressure to at least start bleeding my motorcycle rear brake?"
I had already solved the problem and thought ill let others know theres a solution and maybe my guess as to why it works, I know how frustrating it is and I never buy a complete master cylinder!
I joined here and put how I solved it but I did not pick up that was the same exact problem many others had here, it was the starting of the process of brake bleeding not the loosing of the ability to keep up a functional brake. So as I said (and its well hidden) I fitted a 5 psi connection to the top of the master cylinder and it worked (meaning it got me that first pressure) but there was an easier step where I got the first pressure cycle....when I used the syringe to prime.
I omitted when using the 25 mm diameter syringe filled with brake fluid to prime the separate disconnected parts from rear caliper to the master cylinder (which i took off and split a bit and rotated in space while pumping in fluid and tightening the two halves although looking back probably not needed) anyways i refitted the caliper now primed attached the rear brake line put the syringe at the master cylinder end pumped in fluid whist tightening the caliper banjo bolt as fluid came out so now the brake line and caliper were filled.
I filled the master cylinder and attached in to the bike coupled up the end of the brake line and whilst still loose:-

"Forced in brake fluid fluid by attaching the syringe to where the master cylinder goes"

The end of the syringe fits perfectly into the plastic inlet on the master cylinder so as you press the syringe it seals and pushes in under good pressure, all the time i tried to operate the brake and noticed when i was pressing even lightly on the syringe and on the very first stroke the brake worked perfectly straight away and i mean perfect.
I continued and nipped up the master cylinder to brake line banio as fluid came out (lots of mopping up with water).
I guess this was the ureka moment where I thought if i can maintain a pressure I can get to bleed the brake so I fitted the 5 psi air pressure feed to the master cylinder cap (you may be able to see it in my pic) thinking ill bleed right from the resovouir down
When I did get to bleed under 5 psi assistance no air that I was expecting due to the rubbish performance of the brake came out the back so likly it did not need bleeding but the brake was solid it worked, (just like when the syringe was on).
In the end I used the 5 psi to get the 5 KG weight on the lever and left in for 3 days.....and bingo.
But looking back.....theres no need to pressure assist bleed from the resaviour, lets face it if that goes pop brake fluid everywhere!

Long story short use the syringe to both prime and then keep a small pressure on to get the initial brake solid feel and pressure then load up the rear brake with weight an/or bungies leave for 24+ hours you will have a functional brake.
Reattach the master cylinder any bubbles will naturally find their way up to the resaviour that side is not part of the pressure system just dont let them get in and if its like my Raptor you have rear brake again!
I hope this helps, if it works let me.
 
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I've never looked into it but thought perhaps the groove in the slave cyl was slightly tapered, giving a leading edge to the seal the same way a directional seal would. True that even if seals look excellent, they can fail to function properly. The Sumitomo blue-spot R1 calipers I use were binding slightly on release - i rebuilt with new seals - the old ones looked and felt perfect - the new ones resolves the problem.

I also wondered if tying the lever to the bars could aid seal expansion if there's no pressure to start with; the seal will just be sitting further into the m/cyl bore but be no different in shape from the fully released position.
 
OK - I understand now. Get some pressure behind the lever any way you can (the syringe idea seems effective) then load the lever and leave for 24 hrs. Most on here (including me) have been unable to get ANY pressure, but if the syringe in the feed port works it'll save hours of frustration, huge amounts of Alien's blood mess (our family's name for brake fluid :ROFLMAO: :eek: ), and wasted energy (not to mention unnecessary new m/clyinders!).
 
piranha-bro2 said:
I've never looked into it but thought perhaps the groove in the slave cyl was slightly tapered, giving a leading edge to the seal the same way a directional seal would. True that even if seals look excellent, they can fail to function properly. The Sumitomo blue-spot R1 calipers I use were binding slightly on release - i rebuilt with new seals - the old ones looked and felt perfect - the new ones resolves the problem.

I also wondered if tying the lever to the bars could aid seal expansion if there's no pressure to start with; the seal will just be sitting further into the m/cyl bore but be no different in shape from the fully released position.
Click to expand...
I think it all cases loading up the brakes overnight deforms all seals to adopt the perfect "set" so improves things all round, moto x boys swear by this trick.
 
As long as any air within the system is trapped and cannot escape, no amount of pumping at the lever will budge it. The amount of fluid moved by a single stroke of the lever is usually far too small to effectively move a large air bubble along the brake line. This method may work OK on cars with a much larger M/C moving maybe 2-3 times as much fluid per stroke, but the tiny amount of fluid shifted by the common for us Brembo PS14/15 M/C is pushed straight back into the reservoir by the pressure of the compressed air bubble or even just the to-and-fro movement of said bubble.

Fill the system with fluid and place the bike so that there are no traps for air bubbles along the path of the brake lines (loops, splitters, switch housings, etc), ie, make sure any trapped air can rise all by itself. If necessary, lean the bike against a wall, keeping an eye on the angle between M/C and brake line exit. In many cases, I've simply removed the entire braking system en bloc and rigged it up on the workbench. Once air can find its way to the M/C junction, it's mostly just a matter of moving the M/C piston seal just past the bleed hole of the reservoir and bubbles will escape into the reservoir. A wooden wedge inserted between the caliper pistons prevents them from popping out. In 98% of all cases, I have been able to attain a decent pressure point within minutes, even with the system still installed on the bike. In the rare cases this has not worked, success came around after the M/C recieved new seals. Another common fault is an incorrectly installed M/C piston that either does not cover or free the bleed hole to allow fluid to be pushed into the brake line. Many RGS/SFC1000 rear M/Cs fail as soon as their piston is moved beyond their normal working range, a rebuild with new seals is seldom successful. Unfortunately, replacement of the M/C is then the only remaining option. The pumping bleed method should be avoided on these. If the M/C worked up to that point, the first stroke that pushes the piston further than usual may destroy the M/C! I don't have a viable explanation for this phenomenon, just that I've experienced it many times. Thankfully, the box of rear M/Cs was well-filled back then. ;)

I generally use a vacuum tool to fill/renew brake fluid, rarely takes longer than 10 minutes to get a rebuilt system working as it should. Same tool can be used to quickly replace fork oil, drawing oil from the drain screw into the slider, provided the top stanchion seal has provision of some sort for attaching a vaccum hose.

New caliper seals can restore "feel" at the lever in many cases, the wooden feel of many Brembo systems result from the tiny amount the pistons retract once brake pressure is relieved with old seals. New, pliable seals will retract the pistons much further, resulting in a little more travel at the lever with a far more pleasant action.

I've never bought the "tie back the lever" trick. All it does is pressurise the system and if the M/C seals work as they should, nothing at all will pass them, in neither direction.

piet
 
sfcpiet said:
As long as any air within the system is trapped and cannot escape, no amount of pumping at the lever will budge it. The amount of fluid moved by a single stroke of the lever is usually far too small to effectively move a large air bubble along the brake line. This method may work OK on cars with a much larger M/C moving maybe 2-3 times as much fluid per stroke, but the tiny amount of fluid shifted by the common for us Brembo PS14/15 M/C is pushed straight back into the reservoir by the pressure of the compressed air bubble or even just the to-and-fro movement of said bubble.

Fill the system with fluid and place the bike so that there are no traps for air bubbles along the path of the brake lines (loops, splitters, switch housings, etc), ie, make sure any trapped air can rise all by itself. If necessary, lean the bike against a wall, keeping an eye on the angle between M/C and brake line exit. In many cases, I've simply removed the entire braking system en bloc and rigged it up on the workbench. Once air can find its way to the M/C junction, it's mostly just a matter of moving the M/C piston seal just past the bleed hole of the reservoir and bubbles will escape into the reservoir. A wooden wedge inserted between the caliper pistons prevents them from popping out. In 98% of all cases, I have been able to attain a decent pressure point within minutes, even with the system still installed on the bike. In the rare cases this has not worked, success came around after the M/C recieved new seals. Another common fault is an incorrectly installed M/C piston that either does not cover or free the bleed hole to allow fluid to be pushed into the brake line. Many RGS/SFC1000 rear M/Cs fail as soon as their piston is moved beyond their normal working range, a rebuild with new seals is seldom successful. Unfortunately, replacement of the M/C is then the only remaining option. The pumping bleed method should be avoided on these. If the M/C worked up to that point, the first stroke that pushes the piston further than usual may destroy the M/C! I don't have a viable explanation for this phenomenon, just that I've experienced it many times. Thankfully, the box of rear M/Cs was well-filled back then. ;)

I generally use a vacuum tool to fill/renew brake fluid, rarely takes longer than 10 minutes to get a rebuilt system working as it should. Same tool can be used to quickly replace fork oil, drawing oil from the drain screw into the slider, provided the top stanchion seal has provision of some sort for attaching a vaccum hose.

New caliper seals can restore "feel" at the lever in many cases, the wooden feel of many Brembo systems result from the tiny amount the pistons retract once brake pressure is relieved with old seals. New, pliable seals will retract the pistons much further, resulting in a little more travel at the lever with a far more pleasant action.

I've never bought the "tie back the lever" trick. All it does is pressurise the system and if the M/C seals work as they should, nothing at all will pass them, in neither direction.

piet
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Totally agree the tie back method makes no sense if the voodo explanation is "it gets the air out"as there is no where for it to go its by definition a closed system but after weeks of trying it instantly worked and stayed working, I belive it deforms the master cylinder lip seal to an optimal "set". either way it works.
 
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