Some of you may have been following my posts on brake sticking problem with my S1 Coupe. After much time and effort I tracked the problem to the replacement master cylinder. The two nylon bushes were reversed.

I purchased a rebuild kit from SNG Barratt. The kit is from Caparo Breaking Ltd (yes, it's spelled breaking). The instructions supplied with the kit show to install them backwards.

You can't see it in the Parts Catalogue but if you look at the cross-section view in the Bentley manual you'll see that the secondary cup seal is supposed to nestle in the groove in the longer nylon bush. It can't if it's assembled in reverse. Also the o-ring on the o.d. of the longer bush is supposed to be the outermost seal.

I think what happens is as the piston travels inward one side of the secondary cup collapses into the groove in the longer bush. This causes the shorter bush to cock. Once it cocks the main spring isn't stiff enough to pull it back straight, so the piston is prevented from travelling outward the last mm or so. This leaves just enough pressure in the system to cause the brakes to drag.

It's temperature dependent because then nylon bush has a higher CTE than the cylinder. The return spring can pull it back into alignment when it's cold but not when its hot.

Also, when assembled backwards the entire assembly is longer by the depth of the secondary cup so this may interfere with uncovering the return port to the reservoir.

So beware of new master cylinders labeled Lockheed and supplied by Caparo. They may work properly at first but not for long.

I've notified SNGB of the problem.

Submitted by mr.jwhey@rocke… on Mon, 07/14/2014 - 15:02

Alan,

It's nice to know I'm not the only one w/ this problem. So far they haven't asked to examine my old unit. To be fair to SNGB- I bought the first one from Parts Geek; but it was made by Caparo.

XKs Unlimited claims to manufacture their own. Is that what you bought?

J

Submitted by alan.barc@veri… on Mon, 07/14/2014 - 13:02

I bought a "Lockheed" brake master cylinder from SNG two years ago and had progressively worse front brake dragging, despite going through all the other replacement/rebuild efforts. Last year I bought another BMC from a different supplier (original source unknown to me) and so far so good. I have asked SNG if they want to examine their unit or have me do so and report my findings. They will let me know after their review with Caparo.

Submitted by mr.jwhey@rocke… on Mon, 07/14/2014 - 12:15

Bill,

SNGB has alerted Caparo but I've yet to hear what their response is.

First I purchased a seal kit which showed bushes reversed. I rebuilt mine the correct way using the same bushes, which do not show visible wear, and the piston still hangs up with few millimeters remaining. Took it apart inspected again and noticed small wear marks on the piston head. No visible wear in the cylinder bore.

So I purchased a new Master from SNGB and the instructions show the bushes reversed. I've asked them to advise whether or not to disassemble it and reverse the bushes. I'll post whatever I receive for response.

Submitted by redbil@aol.com on Sun, 07/13/2014 - 13:14

Joseph,

Thanks for the heads up; I installed a Caparo master cylinder last week and it appears to be working but if there is an issue, I should talk to SNG Barratt now. Can you clarify: is the problem with a new cylinder, the repair kit or both?

Bill Braun