upgraded master cylinder binding brakes

[Bikergonebald]

Hi Guys,
I'm hoping someone may be able to help me with a problem on a Reliant Scimitar SE6a that's had a brake upgrade. I bought the car recently and I'm sure the guy i bought it from said the master cylinder had been replaced with one from a Rover. Searching on the internet i've discovered that the standard Lockheed brake system has a lot of similarities to a TR7 and SD1 and i've seen on this forum that some have upgraded their brakes using SD1 master cylinders so perhaps you can help.

Most of the brake components on my car are new within the last year so although i need to check everything i think my problem is likely to be the brake balance valve (same as TR7 SD1) or most likely the master cylinder.

When the brakes get hot the front brakes start to bind and the pedal gets harder and higher which i think is the fluid not being able to return to the reservoir when the master cylinder retracts or if the fluid expands from heat. This could be caused by two things.
1. The passage through the cylinder back to the reservoir is blocked.
2. There is insufficient clearance between the pedal and the master cylinder piston so it can't get back to the point where the fluid can flow backwards.

I've seen a lot of posts on various forums where the 2nd fault is the cause so i want to focus on this and ask for your help. I understand that when the SD1 upgrade has been done on TR7's there is talk about checking clearances and shimming or machining. Is this clearance between the master cylinder piston and the servo rod? I do have some free play between the pedal and servo rod.

How do i check and adjust the clearance?
what should the gap be?

Scimitar workshop manuals don't show enough detail on how they are put together and on the Scimitar forum no-one else seems to have done this modification so the advice i'm offered is all about checking the calipers, which i will do but i don't think that's the problem.

Any help would be gratefully received. A diagram showing how it all works would be brilliant.

Thanks
Kevin.

[FI Spyder]

How new are the flexible brake lines? Old brake lines can get soft inside collapsing. The force of applied brakes can force the fluid through but the return pressure isn't enough to allow the fluid back through to relieve the pressure. If replacing brake lines I would recommend spending a little extra and get the braided steel lines as they are better at not expanding and allowing the full force of the fluid to get to the cylinders/calipers (more solid feel to brakes).

[skertonman]

I take it the brakes didn't bind before the change.

When they do bind, is it all or just 1, that would narrow it down to a collapsed flexi hose leading to the binding brake.

Personally I think the actuation rod from servo to master cylinder is too long, giving the impression that the brake pedal is being pushed down slightly the whole time.

As a temp trial fix I'd try putting some washers between the servo and master cylinder as a spacer to test whether the brakes still lock on, if they don't then it's a case of rod adjustment.

[Bikergonebald]

Hi Skertonman,
Thanks for your reply. Because I only bought it recently I don’t know how it was before or if it’s ever been right since. I suspect the ms / servo adjustment because both fronts are affected the same.
I will do as you suggest with spacers. If I need to adjust it properly is it adjustable or do I have to get something machined?

Thanks
Kevin

[skertonman]

Kevin,

I can't recall off the top of my head if the TR7 servo rod is adjustable, but I have had others where the rod that works the master cylinder is adjustable.

[Hasbeen]

When you get the binding problem, crack the bleed nipple on one side. If the problem is fluid return, you will get a small squirt of fluid, & the binding will stop. Does it stop binding on both sides?

Drive until it binds again, then crack the other bleed nipple.

If cracking one bleed releases both brakes, you know the problem is in the pressure distribution valve, or the master cylinder. If only that brake is released you know the problem is in the callipers.

This should give you a guide to where your problem is.

We found that Mazda used a plastic guide in their master cylinders at one time, that would slowly swell. After a decade or so it would jam the piston enough to cause high drag, or even binding. There are always traps laid for the home mechanic by old cars.

Hasbeen

[Bikergonebald]

Yesterday I got the brakes hot so they were binding, I slackened the nuts holding the cylinder to the servo and the brakes released.
There wasn’t anything to adjust so I put in a metal ring that was 1.8 mm thick. They didn’t bind but the pedal felt a bit low so I made up a ‘gasket’ from 1mm aluminium.
Now it all works fine.
Thanks guys.

[skertonman]

Well done that man.