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MG MGA - Master cylinder operation question


I am having lost of difficulty with the new TRW master cylinder on my MGA. I can’t seem to get the clutch to work, and I can’t push anything but a few drops of fluid either upward from the slave cylinder or downward from the Master Cylinder. It looks to my inexperienced eye like the MC isn’t letting fluid past its clutch-side piston.
Looking down on the top of the removed and empty MC, there are 2 holes from the fluid reservoir, one small in the middle of the piston bore, and one large, near the push-rod end of the bore. Both holes are clear and unblocked.
My question is where should the rear ridge of the piston be when at rest? I attach a drawing to show 2 alternatives, labelled A and B. Which is correct?
My rear piston ridge, at rest, is neither one. It is half-way over the large hole, which likely explains why I can’t get fluid moving through the MC.
(By “at rest,” I mean the front surface of the piston is flush with the flat front face of the master cylinder. If I put a thick gasket, with large cut-outs for the piston heads, my rear piston ridge covers about 80% of the hole from the reservoir.)
Your advice would be much appreciated! Mike Whalen
M.E. Whalen

Master Cylinder operation question.
I am having lost of difficulty with the new TRW master cylinder on my MGA. I can’t seem to get the clutch to work, and I can’t push anything but a few drops of fluid either upward from the slave cylinder or downward from the Master Cylinder. It looks to my inexperienced eye like the MC isn’t letting fluid past its clutch-side piston.
Looking down on the top of the removed and empty MC, there are 2 holes from the fluid reservoir, one small in the middle of the piston bore, and one large, near the push-rod end of the bore. Both holes are clear and unblocked.
My question is where should the rear ridge of the piston be when at rest? I attach a drawing to show 2 alternatives, labelled A and B. Which is correct?
My rear piston ridge, at rest, is neither one. It is half-way over the large hole, which likely explains why I can’t get fluid moving through the MC.
(By “at rest,” I mean the front surface of the piston is flush with the flat front face of the master cylinder. If I put a thick gasket, with large cut-outs for the piston heads, my rear piston ridge covers about 80% of the hole from the reservoir.)
Your advice would be much appreciated! Mike Whalen



M.E. Whalen

Check out MGA Guru website re problems and solutions to TRW master cylinder problems.
Alan Anstead

TRW MC's have been an issue for quite some time and disc brake dragging due to MC not releasing residual pressure, bleeding problems and/or leaking were common issues. Two MC's which seem to be OK are those made by AP Caparo (pricey, but its the Lockheed successor)), and those sold by Scarborough Faire, aka NOS Locators on ebay. My preference has been to resleeve original MC with brass, but that's just me!
Nick Kopernik

I too had problems with the brakes dragging, even after several bleeding sessions. Found that the solution was the push rod did not have sufficient play...two simple turns anti clockwise on the rod was enough to release the residual pressure, and all good now.
Gary Lock

Thanks for your comments, everybody!

It is the clutch side that I'm having trouble with. What is the consensus about whether picture A or B is correct? [See the second post from me, where the drawing was successfully attached.]

I've read everything written on every MGA discussion board that I know of without finding an answer to the above question. Am I missing something?

Lastly, my piston at rest is still covering the rear 1/2 of the main return hole. I have very little room for adjustment at the yoke end, and it doesn't look like I could adjust the rod enough.

Happy motoring, Mike Whalen
M.E. Whalen

Bottom line is that the rear face of the piston should touch the rear cover plate gasket.

As others have said TRW does not have a good write up. I went AP Caparo. As I understand it they took over the tooling from Lockheed. However, the operatives took a lot more care with the machining and tolerances. I had both Lockheed and Caparo units side by side on my bench a few years back and the difference in machining was clearly obvious even to the naked eye.

Steve
Steve Gyles

Steve: Thanks! Don't you mean the front edge of the piston should touch the gasket on the inner face of the front cover plate when at rest?

I have that in my at rest position, but the rear flange of the piston still covers 50% of the fluid return hole.

Some have suggested using a very thick gasket, cutting out the centers so that the front face of the piston touches the inner face of the cover place when at rest.

I've done this, and the piston still doesn't move far enough forward to completely uncover the fluid return hole.
M.E. Whalen

I call the back of the piston the end that has the hole for the pushrod. I guess it's all down to interpretation. I call the back of MC the end that has the pushrods and the front end that pushes out the fluid rather than its orientation on the bulkhead.

Steve
Steve Gyles

This thread was discussed between 13/10/2021 and 14/10/2021

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