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MG MGB Technical - Now brake bleeding problems!

Hi,
I have checked the (many) entries in the archive on this point but I'm still confused.

After noticing that one of my caliper pistons was sticking on my 1968 GT, I decided to renew both calipers. I sourced new calipers, pads, braded front hoses and a new piece of brake pipe for the left front brake, from Moss.

All fitted OK, and then I came to bleed the brakes. Started at the rear left per the manual, working round from the back to the left front and then to the front right.

But after many times trying to bleed the brakes using an eezibleed, (which I’ve used on my 1953 TD for years), I cannot get firm pressure at the pedal.

The symptoms are - immediately after bleeding the pedal is tight then after about 3-4 minutes without my pressing it, it fades to virtually nothing. If I pump I get pressure, then it fades. There are no leaks and the brake fluid in the reservoir remains at a constant level.

I am stuck and hope you guys have some idea.
Since the pressure on the pedal fades away, and there are no obvious leaks, do I have a seal problem in the new calipers, or perhaps the master cylinder which would be odd since I haven't disturbed it, rather than air in the system?
Thanks in advance.
Mike
Mike Christie

Check to see the pads aren't retracting once the pressure is released. This can happen when the pistons are not sliding through the seals properly. The pistons compress the seals rather than slide through, then when pressure is released the compressed rubber acts as a spring, pushing the pistons back into the caliper as the seal reverts to its original shape.
It takes only about 20 thou per piston to use up the whole stroke of the master cylinder.

Get someone to operate the brake while you watch what happens, and if possible measure the gap between pad and disc with a feeler immediately after releasa then a few minutes later to see if there is any change. Measure a pad at a time, no point in getting stressed trying to do all at once!
Paul Walbran

Mike, I had exactly the same problem some years back. Here's what I did after fitting a new servo, new calipers, brake pads, pipes and hoses:

bleeding the brakes was a nightmare. Using a gunson eezibleed I bled the brake, staring at the rear nearside, then near offside brakes and then moving on to the nearside brake calliper and finally the offside brake calliper (nearest the master cylinder). So no problem there you would have thought?
When I tried the brake pedal, it was spongy, but pumped up hard after one or two stokes of the pedal. Thinking there must obviously still be air in the system, I then went through the whole process again, with still the same result – spongy brakes, but after one or two pumps, they were fine. So I still had the spongy brake pedal problem.
By now the car had a least 3 litres of new brake fluid pumped through it by the eezibleed and there was obviously still air in there somewhere. I had read the tips about cracking open a brake calliper whilst your “Attractive Assistant” pushed the brake pedal down as hard as they could and I had also read a tip of bleeding a remote servo by bleeding with the engine running and again cracking a calliper bleed nipple, but this time pushing the brake pedal down quickly as the bleed nipple is opened (the idea being that the additional pressure generated by the servo would force out the air that is trapped in it).
I decided to try the latter method and bled the calliper nearest the servo first with the engine running, but without using the eeezibleed. This worked a treat. When I opened the calliper and my “Attractive Assistant” quickly depressed the pedal a great lump of air came out. I then repeated the process on the offside calliper (nearest the master cylinder) and got no further air out.
I then tried the brakes – Eureka, a firm brake pedal at last. The car was MOT’d the following day and passed with flying colours.

Also, have a look at Paul Hunt's website, which is very helpful

http://www.mgb-stuff.org.uk/braketext.htm#bleed

Hope this helps.

Andy

Andy Robinson

Mike, If your B is fitted with the remote servo, this is often the problem. It has been recommended that the servo is rotated on it's mounting plate so that the air valve is at 5 o'clock.
I improved matters as regards bleeding the servo slave by fitting a bleed nipple to it. It just requires a double banjo fitting. This is a Harley Davidson part I think.........hence the chrome!


Allan Reeling

Absolutely standard.

You need a high-pressure step after the basic low-pressure bleed.

Get someone to press down hard on the brake pedal while you rapidly open and close each calliper nipple in turn, and I've always found this blasts an extra 'lump' of air out. After that they are fine. The front of the car usually needs to be higher than the rear during any bleeding.

The reason you can 'pump up' the brakes is because of the 'slow-release' valve (http://www.mgb-stuff.org.uk/mcvalve.htm) that is fitted to the brake master. This opens to allow full fluid flow when you press the pedal, but closes when you release to only allow the fluid to trickle back.
paulh4

Thank you for all your advice.
When my wife gets back from walking the dogs we'll have a go at pumping the brakes and opening and closing the caliper nipple.
I'll let you know the outcome!
I am much obliged.
Mike.


Mike Christie

Had same problem after fitting new servo on my '73 BGT , followed Paul's (excellent as usual) advice but was only 99% happy, so I got the attractive assistant to push hard on the brake pedal while I cracked open the top servo union that extra "lump"that was released made me 100% happy & still am.
Bill




W M Griffiths

Update.

After trying all the range of suggestions I still had the problem of a 'double pump' to get brake pressure.

So I took the car, (very slowly) to my local MG garage who tested the system and pronounced that the calipers were faulty. They had come across this problem before with calipers supplied by Moss.

Paul Walbran was right, the tolerances between the piston and the seal in the Moss calipers are such that the seal acts much like a rubber band, not allowing the piston to move freely.

The garage has found that calipers supplied by the MGOC are built to different tolerances and they have not had this kind of problem with them.

So new MGOC calipers were fitted and the problem cured.

Thanks again for all the help.
Mike.
Mike Christie

Mike, you could try and lubricate the pistons by removeing 1 pad at a time and pumping the piston out ( not all the way), then pushing it back in. Do this a few times on all the pistons. That might help. Hopefully the pistons are not rusty!
Paul W is correct about the seal gripping the piston when there is excessive pad to disc clearance. So you end up with a long pedal travel. The seal is meant to deform when the brake are applied and retract the piston when the brakes are released, but push through the seals when the seal can no longer deform any more, it's a very simple auto adjustment.
The problem with classic cars that some don't do a lot of miles, so this problem can be an issue.

regards John
john wright

Where I work regularly (around every other car we build) has to do what John Wright says on Moss calipers to get them to work properly. Usually one pump-out per piston is fine and everything is ok from there out.
Never had any issues the calipers once this (admittedly annoying) first step is taken.
Roadwarrior

Mike, it's interesting your comments on the MOSS supplied calipers. When I fitted the new calipers to my car, they were supplied by MGOC Spares and I had exactly the same problem with sticking pistons. I solved it (as others have said), by pumping the pistons 3/4 of the way out and lubricating them with caliper piston grease.

I then rebuilt the originals to keep as spares as I felt that the quality of the new calipers left a lot to be desired. The old story of the quality of manufacture of replacement parts.............

Andy
Andy Robinson

This thread was discussed between 31/07/2016 and 05/08/2016

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