Brake Pedal Pressure

[Cammsjb]

Can anyone suggest why I have to press the brake pedal a second time before I can feel any pressure at the pedal, I think I have purged all the air out of the system the best I can, my car is a MK2 'S' fitted with refurbished standard servo.

[AndyPen]

Sounds as if something must still be compressing I'm afraid, and that something is almost certainly air.

http://mk1-forum.net/viewtopic.php?f=3&t=28500

[bwaminispeed]

Rear brakes un adjusted can cause the same effect.......

[Peter Laidler]

A little point that people forget - or don't seem to know about - is that there is a very small hole leading from the outer LOW pressure brake fluid tank and the inner HIGH pressure cylinder. As soon as the piston passes this hole........ anyway, that's enough of the physics lesson. But if this hole is blocked OR PARTIALLY BLOCKED with crud, you are limited to using the fluid that is available and on release of the pedal the high pressure part of the fluid line doesn't recuperate in time. Next press of the pedal and you get a slack feel THEN pressure

Exactly that was causing this for me until I sat and worked it out. I detailed this in a technical article I wrote that's somewhere in the tech pages as I recall, together with a small right-angled tool for clearing the crud (it was black rubberised sulphur from the seals and hoses that'd solidified in my case) he problem was solved.

There's also a a school of thought that you can compress brake fluid - or any liquid!! You can't, it's a basic law of physics. As Andy says, if you've got a spongy pedal, you're compressing something. And it ain't liquid.

[850man]

9 out of 10 times it will be the rear brake adjustment.
If the drums are machined or worn out replace them, along with new shoes. Same with the front disc rotor thickness.
Some of the new S type replacement callipers can be a pain to bleed properly as well, you have to push the pistons all the way in while bleeding or they seem to hold air. It doesn't happen with the original callipers.

[andy1071]

Peter,
The hole you refer to is the "cut-off port" (it was actually pierced outwards from the bore, during production, so it didn't have a burr to damage the seal).
This hole releases the pressure in the brake system when the pedal is released
(not forgetting the non-return valve in the end of the cylinder that holds a slight residual pressure -about 4psi- in the system to prevent caliper piston "knock-back").
The fluid that 'fills' the system comes over the primary-seal as it 'collapses' as the piston returns (via the larger hole from the reservoir, 'behind' the cut-off port).
So if the cut-off port is blocked, when you repeatedly press the pedal, you tend to build up the pressure in the brake system, and it can't release.

All this is also dependent on how old the seals are... As you know they tend to soften with age, which starts to mess-up the function.

The black stuff that dis-colours the fluid and collects in the reservoir is "carbon black".
Carbon powder that is part of the rubber compound that 'leaches out'.
-A sure sign that the system is "old"...

Also, one of the main 'problems' with the Master Cylinders in a Mini is that they are vertical. So there is a tendency to 'gulp' air into the system when the pedal is released.
This is not usually a problem when everything is new, but with time and age, it can start to occur.
-The vertical cylinder means that any air entering the cylinder won't vent out through either hole, and tends to enter and build-up in the system....

[Peter Laidler]

You are correct Andy. I fully understand the mechanics and hydraulics of the system but when it happened to me, I was blind-sided until I set about it - and solved it!. Yes, the black gunge..... It mystified me

Yes, the black stuff is indeed carbon from the seals and hoses. I was in a position to analyse this black gunge in the uni labs here over the weekend and was quite shocked by the result. Maybe writing up about spring technology earlier caused 'sulphur' to stick to my brain cells!!!!!!

But if reminding the forumers that the cut-off port is an important part of the braking system, then we've done our job.

Regards from Abingdon, the ancestral home of MG Cars

[Cammsjb]

Thanks for all advice within the posts, I will work through all the possible causes and hopefully one or more will cure the soft pedal issue, checked the whole system for leaks today and all connections are dry.
I didn't mention in my first post that all of the brake system has been replaced with new pipes and fittings etc. except for the servo which has been refurbished, the next check will be the rear brake adjustment.

[agricola]

Peter and Andy are as thorough as usual, and no doubt worth checking out. May I recount a problem I had with another bleeding problem on a big industrial loader, master cylinder servo powered by a diesel engine exhauster and 4 slaves. Basically any tiny amount of air amplified the problem. So in desperation I removed the linkage and put clamps across the slave cylinders so they were closed, absolute minimum volume and no flexing, that normal did the trick I have also done it on disc brake callipers and I think it works

[AndyPen]

Somewhere on here I have put pics of my new master cylinder, which as it turned out was one of the bad batch. I never did find out exactly what the issue was with these, but it most certainly didn't hold any pressure.