Morris Minor Owners Club

My car has just been put back on the road, standard system with all new pipe's and shoe's, wheel cylinder's and a good quality master cylinder. I've used silicone fluid with no problems, doesn't pump up, brakes are good and is not spongy. However when the brakes get warm / hot, all four drums start binding, like having the brakes half on, there is some slack at the peddle when cold about half an inch, which disappears when hot?.

You should have read all the thoughts about Silicone brake fluid.... I'm not sure that's the problem - but it may be... I believe you can just change to normal brake fluid without any problems - the opposite however from normal to silicone cannot be done. Is there a servo in the system?

I did read some of the other comments on using silicone BMC, and I did say it was a standard system, (no servo) . I'm reluctant to change as I like the idea of rust free cylinder's etc...just seems like silicone expands more when things warm up.

I also run silicone in a standard system and haven't had the problems you describe. The brakes do fade in steep hills but that's probably more due to spirited driving than the brake fluid.

Your problem could be due to small air bubbles in the fluid, not big enough to be noticeable when cold but might expand when warm and cause the binding. According to the workshop manual the brake pedal slack should be about 3/4" so I'd try that first and maybe change the fluid if that doesn't work.

Silicone fluid must be poured very slowly and carefully to avoid air bubbles; but if it isn't dirty the one already in your system can be left to stand so that the air bubbles disappear and be reused.

It may just be the 'retained pressure' in the system holding the shoes on slightly. Perhaps try slackening back the shoe adjusters slightly ? I was just checking about the 'servo' - some would perhaps think it was 'standard' if it was on the car when they acquired it....

David,

If there is a pressure build-up in all the cylinders, it means that the fluid is not able to return to the master cylinder reservoir, through the by-pass port, to prevent any pressure build-up.

Lift out the carpet and underlay, remove the transmission cover plate for the master cylinder and remove the m/c filler plug. GENTLY press the brake pedal while looking into the reservoir, there should be a small fluid disturbance after the free play has been taken up. DO NOT press the pedal quickly, or you could get fluid exiting the m/c and dribbling down the side and making a mess, or worse going into your eye!. Do you get the little 'plop' in the fluid? If you do then it looks like the free play adjustment is about right.

Repeat when the problem occurs - is there any difference? If there is now no fluid movement the most likely cause is that the main cup seal is covering the by-pass port. Arrowed in red below:
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There are some likely causes:

- The pedal is not returning, so preventing the master cylinder piston from returning fully. Is the return spring in place? Is the pedal free to rotate?

- Insufficient free play between the pedal and the master giving the same result - the m/c piston cannot return fully.

- The m/c piston main cup seal is swelling up and covering the return hole - but this would be permanent I would think??

I suppose the pressure build-up could be a faulty residual pressure valve at the base of the master cylinder? I have not heard of it, but if it allowed significantly more than the designed 8 lbf/in2 pressure to accumulate, that would start to apply the brakes, not just keep the wheel cylinder seals in shape?

Something looks to be changing as the brakes are used. Check the free play first and work from there.

Could be small air bubble's in the system, but they would have to be small as the brakes don't pump up at all, also I can't slacken off the adjustment any more because they are at the lowest setting now.Yes there is a small plop of fluid, checked that when making sure the cap hole was clear, I must admit I did have difficalty getting enough play at the peddle, I will look at the fluid any again as you say, then maybe I'll have to shorten the rod a little to get more free play.

I do hope this is not the case, but some rubber seals don't react well to the silicone fluid and swell up; which could also cause friction inside the cylinders and make the brakes rub (but I imagine this would happen at all temperatures and not just when warm).

If using silicone it's best to leave the seals soaking in fluid for a couple of days to check if they swell up; if yours have it would be a matter of either replacing the seals or going to regular brake fluid.

Another problem with my Minor brakes now, cured the binding and adjustment troubles... now not having used the car for a few weeks the peddle almost went to the floor, did pump up a bit so bled it all again starting with the longest brake pipe, I did hear air bubbling out a little into the jar via a non return valve bleeder tube, a few on the offside rear and near side front.

It's better again but still pumps up, checked prior to this for leaks at all the joints, none and the master cylinder was overfull so why did the peddle go almost to the floor?, I started all this with a completly new system using Girling parts and synthetic dot 4, maybe the seals in the system don't like the synthetic fluid?, can I go over to the ordinary stuff now or is it a seal change on everything... hope not,

DOT 4 is 'ordinary' stuff. No worries there. It's the best fluid you can have.

Okay, is synthetic fluid not silicone based fluid then, have I been using standard Dot 4 all along, dosen't smell or feel like the brake fluid of year's ago.

Silicone is DOT 5...... All brake fluids are 'synthetic' - but DOT 4 is NOT silicone - thankfully....

Well I didn't know that just assumed it was silicone, better perceiver with what I've got then, thanks.