I thought I would do this post hoping that it may help someone out there.
I have recently completed my 1962 3.8 restoration to the point where I thought I would do a trial spin round the block. This is when a common problem I’ve read about on here showed itself. The brakes not releasing! The rear brakes were dragging and were hot. I had to crack off the output line at the master cylinder to release the pressure so I could limp home. On inspection it was clear that the pressure in the rear master cylinder (top one) was locked in and not releasing once the pedal pressure was released.
I read all the posts on this forum I could find about this problem where a variety of possible solutions / causes were discussed. This post by ‘Rory’ was particularly interesting.... (Thank you Rory)
viewtopic.php?t=4120
It showed the workings of the brake linkages and master cylinder (MC) components in great detail. But what I needed to know was what was happening within the MC as it is operating to see what could be stopping the pressure from equalising. I came across this illustration of a standard MC on-line......
It’s shown in the ‘at rest’ position i.e foot not on the brake. What I noticed is that there is a gap at the back of the MC where there’s a valve (Normally Open). This is the input from the reservoir. So, ‘at rest’, there should be no internal pressure as it should be open to the reservoir which is at atmospheric pressure.
With my MC off the car (and drained of fluid) I figured if I blew down the outlet the air should come out of the inlet (due to the gap) and vice versa.
It didn’t! Now all I have to do is find out why!
After having it in bits for half an hour and trying to figure out why my MC had no gap at the bottom, the penny dropped! There’s a concaved washer (‘W’ in the illustration above) that holds the push-rod in along with a circlip. The concaved washer on mine was in the wrong way round!
The consequence of this is that the inner piston assembly is pushed further into the cylinder. Far enough to close up any gap at the bottom. I turned the washer round, reassembled it and all was good. Air passed through the inlet and outlet as it should.
I don’t’ know who made this mistake (probably me) but it just goes to show how something so simple can cause big problems.
Anyway, one of the reasons for the post is that I read that this problem often occurs after people have had their original MC’s refurbished. A suggested possible reason for this was that the new seals they install are not as good as the originals. This may well be true but I wondered if it’s also a possibility that they too have had that push-rod washer inserted the wrong way round when rebuilding them? Might be worth a look if you’re having a similar problem. Cheaper than forking out for a brand new MC!
cheers
Sean
