Brake failure, but I don't understand why

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S

Steve_R1979

Active Member
Posts
634
Location
Horsham, West Sussex
Hello!!!

I've had some trouble free motoring from the RR recently (hardly driven it) but last week I took it out and half way home the brake pedal stopped doing its thing.

First press it goes to the floor, but springs back OK.
If I pump the pedal the brakes come back, but the pedal feels terrible.
The further I drive, the worst the brakes got.

Went back out to it today to check the obvious bits:

Fluid level OK
No leaks around the resevoir or brake lines that I can see
and the strange bit.....brakes feel fine. Pedal feels as good as it ever did, so what on earth is going on???

Any ideas?!

Thanks!
 
Saint.V8 said:
What year??

If it is a later one with the hydraulic Pump and Accumulator - I would say the pump and/or accumulator is shot....

If it is vacuum operated, check the vac pipes and the master cylinder seals....
Click to expand...


Not checked the bleed nipples but will do.

Its an 87.

There is a big thick pipe going from the plenum to the big spherical thingy behind the master cylinder.
 
Spherical?? or Circular??

If it is circular, that is the Brake Servo - works on vacuum generated from the intake manifold....

Check the bleed nipples and all unions for security and signs of leakage - check the vacuum hose isn't split.

Could be leaking past the Master/Slave cylinder seals.

Could be the servo diaphram has split if all else seems well.
 
failed brake servo would give very stiff pedal ,worth pulling master cylinder back a bit and seening if seals are leaking or if reservoir has black deposit at bottom
 
If the pedal goes to the floor then is ok when pulled back it is odds on the NRV that feeds the fluid from the tank. Maybe a bit of **** floating about fouling it.
 
I don't believe this! I came on here to write up a problem with my '85 Classic, only to find Steve has written it for me.

My Classic has behaved perfectly since Classic Kev had a tweak (goes even better with the new diaphragms, Kev), but I'm still trying to figure out how to balance the carbs. They're the ones with an anti meddle cover on the primary throttle stop, and the second carb doesn't appear to have one at all. Just guessing ... do I need to adjust the length of the tie bar to achieve a balance?

Whatever, back to the brakes ... Perfect 15 mile run across the moors ... switched off for ten minutes whilst dropping the Mrs off in Llandernog ... started first pull (It's often a little reluctant when warm) ... drove off OK but had to stop and back up to let a lorry through ... brakes felt spongy stopping in reverse ... Drove on through, testing the brakes gingerly ... Pedal travelled well down towards floor, but there was a braking effect low down ... checked for fluid loss ... Reservoirs full OK ... no sign of leaks on floor or elswhere. Seemed like it was maybe only braking on the secondary circuit ... crossed fingers and drove it home ... slowly.

After about 5 miles, and successfully negotiating the hill on Denbiegh High Street, the brakes seemed to be working correctly again.

Next day, Sunday, after the rain stopped, I re-visited the problem ... checked all hoses and connections ... no leaks found anywhere ... no loss of fluid ... and the pedal still feels solid. Made a nuisance of myself charging up and down the A5, slamming on the brakes, but still couldn't replicate the fault. Everything's all hunky dory again.

Baffled ??? Even more baffled by Kev's remark about checking the wheel bearings ???

Johnny.
 
jwm said:
I don't believe this! I came on here to write up a problem with my '85 Classic, only to find Steve has written it for me.

My Classic has behaved perfectly since Classic Kev had a tweak (goes even better with the new diaphragms, Kev), but I'm still trying to figure out how to balance the carbs. They're the ones with an anti meddle cover on the primary throttle stop, and the second carb doesn't appear to have one at all. Just guessing ... do I need to adjust the length of the tie bar to achieve a balance?

Whatever, back to the brakes ... Perfect 15 mile run across the moors ... switched off for ten minutes whilst dropping the Mrs off in Llandernog ... started first pull (It's often a little reluctant when warm) ... drove off OK but had to stop and back up to let a lorry through ... brakes felt spongy stopping in reverse ... Drove on through, testing the brakes gingerly ... Pedal travelled well down towards floor, but there was a braking effect low down ... checked for fluid loss ... Reservoirs full OK ... no sign of leaks on floor or elswhere. Seemed like it was maybe only braking on the secondary circuit ... crossed fingers and drove it home ... slowly.

After about 5 miles, and successfully negotiating the hill on Denbiegh High Street, the brakes seemed to be working correctly again.

Next day, Sunday, after the rain stopped, I re-visited the problem ... checked all hoses and connections ... no leaks found anywhere ... no loss of fluid ... and the pedal still feels solid. Made a nuisance of myself charging up and down the A5, slamming on the brakes, but still couldn't replicate the fault. Everything's all hunky dory again.

Baffled ??? Even more baffled by Kev's remark about checking the wheel bearings ???

Johnny.
Click to expand...

I've already told you what could cause that. Could be NRV. Or if the wheel bearings are badly worn the disc moves from side to side as you go round corners, and pushes the caliper pistons back in. When you put your foot on the brake the pedal travel is greater as it pushes pads back towards disc.
 
Last edited:
Yep, I can see yer point about badly worn wheel bearings causing the disc to push the pads back in, but my bearings are all fresh off MoT good.

I assume NRV to mean Non-return valve, but where would this be situated? My brake fluid reservoirs are sat directly on top of the master cylinder and from the manuals and spares list I cannot see a NRV in there.

I'm starting to think maybe a total rebuild of the master cylinder could be called for ?

Johnny.
 
Last edited:
I'm back in Belfast today so won't be looking at mine until the weekend.

I'm going to start with the master cylinder. I know my disks are good, and although I have a squeaky wheel bearing there is no obvious play.

The other thing that rules out (in my mind) the possibility of it being the disk or bearing is that when I parked up the pedal was really spongy, but now it's fine so something has changed without the car being move or engine being started.
 
jwm said:
Yep, I can see yer point about badly worn wheel bearings causing the disc to push the pads back in, but my bearings are all fresh off MoT good.

I assume NRV to mean Non-return valve, but where would this be situated? My brake fluid reservoirs are sat directly on top of the master cylinder and from the manuals and spares list I cannot see a NRV in there.

I'm starting to think maybe a total rebuild of the master cylinder could be called for ?

Johnny.
Click to expand...

When i say NRV it is a loose statement. The bleed from the tank is usually through galleries in the piston. And sealed by a metal washer. There are various ways of doing it. As you press the brake the washer covers the bleed holes preventing fluid being pushed back into the tank. If you have a bit of crap floating around it maybe getting between the valve and the piston stopping it from sealing. Had a famous incident of this many moons ago with a brand new Austin 1100. The customer complained of no brakes on occasion. One of the lads after going around the block several times and flying up the workshop and screeching to a halt. Found out what he meant, when on his last try the car ended up nose down in the commercial pit. OH dear. A bit of swarf was found that now and again stopped the valve from sealing. A lesson learned for all. Yes get a repair kit for it they are not out of the way expensive make sure everything is immaculately clean when you put it back together.
 
Last edited:
If I can decypher from the Wammer - I attach a drawing of the Master Cylinder....

I have circled the Seal - when the pedal is pressed this shaft and seal moves in the direction of the arrow and as can be seen this closes off the feed from the tank above and pressurises the hydraulic circuits for the brakes....

If there is FoD in the cylinder by this seal, when it is pushed forward it won't seal the tank feed and it will allow the hydraulic pressure from the piston to push fluid into the tank!
 

Attachments

  • RRC_BRAKE_MASTER_CYLINDER.jpg
    RRC_BRAKE_MASTER_CYLINDER.jpg
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Saint.V8 said:
If I can decypher from the Wammer - I attach a drawing of the Master Cylinder....

I have circled the Seal - when the pedal is pressed this shaft and seal moves in the direction of the arrow and as can be seen this closes off the feed from the tank above and pressurises the hydraulic circuits for the brakes....

If there is FoD in the cylinder by this seal, when it is pushed forward it won't seal the tank feed and it will allow the hydraulic pressure from the piston to push fluid into the tank!
Click to expand...

Yup zakerly that. That is just a rubber seal that covers the inlet. On the secondary circuit the second method i described maybe used. Or it could be just a drip hole that the piston seal goes past on brake application.
 
Last edited:
Ah-ha ! Comprehension dawns. Amazing how a picture can save a thousand words, eh?

So ... the way forward is to strip, flush and thoroughly clean the master cylinder, and as they ain't expensive, whilst it's in bits, fit new internals anyway. After this of course, the brake lines and pistons will need drastic bleeding, so a complete pressure flush through and fill with new fluid may as well be carried out.

Before I start shopping though ... any expert opinions as to which is the BEST Brake fluid to use? Nothing but the best for my baby truck. I'm in love, and I ain't skint.

I learned a long time ago not to skimp on hydraulics, and the importance of clean fluids. I've been running big Citroens on and off for 40 years, and I'm currently trying to find a DS21 to fill the space left by a recently departed Mk2 Jag.

Johnny.
 
Last edited:
jwm said:
Ah-ha ! Comprehension dawns. Amazing how a picture can save a thousand words, eh?

So ... the way forward is to strip, flush and thoroughly clean the master cylinder, and as they ain't expensive, whilst it's in bits, fit new internals anyway. After this of course, the brake lines and pistons will need drastic bleeding, so a complete pressure flush through and fill with new fluid may as well be carried out.

Before I start shopping though ... any expert opinions as to which is the BEST Brake fluid to use? Nothing but the best for my baby truck. I'm in love, and I ain't skint.

Johnny.
Click to expand...
Soon will be...!!:D:p
 
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