13 replies to this topic

[s512]

Hi there,

 

I'm having some problems with the brakes on our 1988 Mini Mayfair (with servo).

 

A few weeks after a new rubber hose was fitted to the offside front, the brakes starting sticking on.  Took off the wheel and opened up the bleed nipple and it became unstuck.  Bled the brakes and there were lots of little bits of rubber coming out (which blocked up the non-return brake bleeding hose I was using).  -> Faulty brake hose I thought, breaking up inside and blocking the caliper so fluid couldn't flow back out.

 

Anyway, switched the front hoses out for braided hoses (forum was a great help), bled the brakes and there was no sign of rubber sediment. But the same thing happens after a drive. Note, it doesn't bind immediately - only after a short drive.  And seemingly only after a drive (pressed brakes many many times while parked up but no locking up happening).

 

The vacuum seems fine - pulled the vacuum hose off the servo and there's plenty of suction there.

 

Another thing I've noticed is pressing the brakes hard really drops the revs - to the point of stalling if it's not quite up to temperature.  Not sure what this could be?

 

So now I'm thinking it's master cylinder or servo. Or could it be something else? Caliper? Is there something I could test before forking out for a replacement? 

 

Thanks for your help!

Sam

[nicklouse]

servo issue is my guess as you say hard press of the brakes causes the engine revs to slow to about stall.

Edited by nicklouse, 04 August 2017 - 08:26 PM.

[Northernpower]

Have you checked the pedal is releasing fully and not sticking and holding the master cylinder on slightly?

[cal844]

To fully test the servo you need to::

Pump the brake pedal so it goes solid

Then start the car whilst keeping the brakes under pressure

The brake pedal should drop towards the floor slightly (maybe 1inch travel).

HTH

[Spider]

I'd say it would most like be the Master Cylinder.

 

If your hoses were at the point where they'd swelled up, then it's most likely the whole braking system is in a similar condition.

 

As part of routine maintenance recommended, the brake fluid needs to be changed periodically and also all the rubber parts of the braking system are recommended to be changed out every (from memory) 40 000 miles.

[weef]

Is it still only one and is it  the same wheel that is binding ?.

[Rorf]

Yes brake fluid should be changed every 2 years, which the majority of vehicle owners fail to do

[slidehammer]

There is a valve in the vacuum hose from the inlet manifold to the master cylinder that could be sticking maybe worth investigating that. You can rebuild the servo unit, I did mine using parts from mini spares and a guide on the internet. Hardest part by far is splitting the unit, once split and the seals lubed with red rubber grease it went back together easily.

I found this guide helpful (even though it is for Triumphs, most of it is the same)

http://www.buckeyetr...Servo/Servo.htm

[Wim Fournier]

By experience I know there is a flaw in the braking system. When you drive the car, the (front)wheels move side wards while you are steering. At these moments the brake pads make contact and by the friction they warm up. So the brake fluid warms up and expands = needs more room. There fore the fluid should go back into the reservoir.  The little holes in the cylinderwall at the foot of the piston(s) should be free to let the fluid pass. Problem with older cars is often that the gasket under the foot of the mastercilinder is flattened over time that much, that the master cilinder sits deeper, so much that the piston in the cilinder does not come far enough down the cilinder to free the little holes that should give way for the fluid to get back into the reservoir.

Buy a fresh gasket and the problem is over.

When you don't beleve it, just put two rings under the foot of the bmc on top of the old gasket. 

You have just to release the nuts enough to put the rings in between, when you use a saw to take a piece out of the ring as wide as the hole in the ring is. Than you can shove the ring in between. Fasten the nuts: The problem is over.

[s512]

Thanks for all the responses!

 

Just double checked it again - sticking on both front wheels - rears are fine. Seems to get worse the more it's driven, I guess that could be pressure building up.  When i open up the bleed nipple, a bit of fluid burps out.

 

The pedal isn't getting stuck down.

 

Tested the servo - pedal went hard with engine off, then switched on and it went down slightly - not an inch though.

 

Haven't checked the valve in the vacuum hose, but the hose is sucking so I assume this should be OK.

 

@wim - are you suggesting this is all I need? https://www.minispar...sic/31G279.aspx

[Wim Fournier]

 https://www.minispar...sic/31G279.aspx

 

Yes.

I have a lhd, so, where the bmc stands there is no plate for the attachement of the 'dogs bone'. So the bmc sits lower against the brake pedal connection. Turned on the nuts a bit fiercely on a old gasket and could not prime the bmc. After weeks of trial and error found out what was cuasing my problem, by reading a post on this forum from a guy with the same sort of problem as you have: blocked openings in the cilinder wall to the reservoir. This enlightened my mind and first I used a pair of rings both sides and my problem was over.

Good luck!

[weef]

A quick try would be to pack the underside with a washer temporarily to increase the clearance. If when opening the front bleed nipples the fluid runs freely, at a reasonably similar rate,  this shows a clear return path to the master cylinder. Just catch the fluid in a clean jar and check for any contaminants, do both wheels at the same time. Remember to keep the master cylinder topped up.

[cal844]

Thanks for all the responses!
 
Just double checked it again - sticking on both front wheels - rears are fine. Seems to get worse the more it's driven, I guess that could be pressure building up.  When i open up the bleed nipple, a bit of fluid burps out.
 
The pedal isn't getting stuck down.
 
Tested the servo - pedal went hard with engine off, then switched on and it went down slightly - not an inch though.
 
Haven't checked the valve in the vacuum hose, but the hose is sucking so I assume this should be OK.
 
@wim - are you suggesting this is all I need? https://www.minispar...sic/31G279.aspx

Servo is working, sounds like either master cylinder or valve on bulkhead

[s512]

Thanks. Just realised that gasket is for the old metal type so I don't think that fix will help.. unless I've misunderstood. I've got the later style master cylinder with plastic reservoir (http://www.minispare...s/GMC90376.aspx)

 

Replacement or rebuild kit it is then I guess.