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MG MGA - Master Cylinder holding pressure

I have recently had a case of my brakes staying on after a short run. It started on a really hot day (for the UK!) 25 degrees. The car was completely rebuilt 6 years, about 3000 miles ago. The master cylinder was a new old stock Lockheed probably quite old as the bores needed some very slight honing to clean some surface rust off but cleaned up perfectly. It was honed by a specialist company. I fitted new seals and used Automec silicone fluid. Everything else was new or restored. Everything worked perfectly till last week and then the problem started. Its Ok to start with but after a mile or two the brakes start binding.

If I unscrew the end plate by half a turn on each screw the pressure releases suggesting that the fill hole in the MC is being covered. So I think I have a solution by packing the end plate off slightly. What is perplexing is that the problem has started now. It might be the compatibility of the seals with silicone but I expected silicone to give less issues with the seals.

With the end plate removed when the brakes are warm the brake piston is proud of the end face of the MC by about 0.5mm but when cooled it is sub flush. Picture when warm shown.

John




John Francis

Have you measured the distance of the fill hole from the face of the MC and the length of the piston from back to seal lip - this should give you a guide to whether it is blocking the hole.
Mike
Mike Ellsmore

John

On my previous NOS Lockheed MC (I now have a Caparo) I found that return hole tended to get covered. I machined about 10 thou I seem to recall off the brake piston face.

Steve
Steve Gyles

Took the MC out last night and found that the seals in both bores stick in the bore and don't come out under the return spring pressure. This is even with the pistons removed. This suggests that the seals have expanded. I haven't got them out yet to measure against new seals.
I do recall that the pistons and seals were much stickier in the bores than I expected when I fitted them. I assumed that they would bed in and get smother.
John Francis

I have dismantled my new master cylinder and compared it with the original one. The distance of the bled hole is almost exactly the same from the front face, 12.25 mm plus the length of the piston. I used a small piece of wire through the bleed hole and dropped the piston against it and then measured to the end of the piston. Measurements varied from 12.00 to 12.30 for both MCs.

The original Lockheed seals are slightly longer than the replacements from Brown & Gammons 10.37 compared with 9.54 and even more interesting is that the lip diameter of the two seals is a massive 1mm different. The Lockheed seal is 23.66mm and the B&G seal is 22.60mm. The wall thickness of the B&G seal is thicker at 2mm compared with 1.5 for the Lockheed seals.

From those dimensions there ought to be enough clearance from the end of the piston and the seal to clear the bleed hole 12.25 - 10.37 -0.22 (for the shim) even with the Lockheed seals. When the seal is in the bore it will get longer as it compresses into the bore but I doubt whether it would get
0.75mm longer. So I am a bit surprised that it all started to hold pressure.

In 30 years I did not have a problem with the original MC.
I'll fit the new seals and see what happens. Has anyone got another seal kit to measure the length and diameter of the main piston seal. The B&G one is only 0.4mm bigger that the bore.

Pictures show new and Lockheed seals. Lockheed on the right.
The springs have the same wire diameter. The only other difference is that the original MC pistons had a chamfer on the seal end face.



John Francis

John

I realise that you have been concentrating your investigation on the possibility of the bleed hole being covered as a result of fitting the new seals. However your last posting suggests that you may be considering other causes.

Did you also change the non-return valve and valve washer? I assume that you did, in the switch to silicone fluid. In that case the problem described by Barney at http://mgaguru.com/mgtech/brakes/bt101a.htm may be worth considering as a possible cause.

I changed my 1960 1600 to silicone fluid last year, replacing all hoses, rear cylinders complete, caliper seals, and fitting a new Bob West supplied master cylinder. The calipers then stayed locked on.

I looked at bleed hole covering as the possible problem, but after extensive investigation including trying other calipers, and after discussion with James at BW, I (reluctantly but desperately) drilled the relief hole described by Barney at the link above. All has been OK since, including on a return trip from England to Sardinia. (Drill size used was 0.028".)

Mike Card
M D Card

Hi Mike

I think the hole is only needed if the return spring is stronger than its needed to be to hold a little pressure without binding the brakes. I don't think this is the case as the brakes were ok for 3 years.
I reassembled the MC yesterday and it was a real job to get the seals and pistons to return reliably to the end stop. The brake side took a lot of working in and out to get it to be free enough. Several times it stuck right at the bottom of the bore. I think it was the seal on the pistons that needed working in. I found the best way to work it in was gently tapping the piston push rod with a mallet. It gave a lot of small movements quickly and was the best way to free it when stuck at the bottom. I am wondering if the silicone fluid is less lubricating than DOT4.

John
John Francis

I have now purchased so AP Caparo seals branded Lockheed from Ebay.

I found that the Brown and Gammons ones bulged over the groove section in the piston as Barney points out and were sticking badly in the bores even after some running in on the bench. http://mgaguru.com/mgtech/brakes/bt101b.htm

The AP Caparo ones are I think much better.

Dimensions:

Main seal diameter 23mm so slightly smaller than original Lockheed (23.66), but 9.4mm long, 1mm shorter than original. This hopefully will avoid the bleed hole getting covered.

The diameter of the piston seal when fitted to the piston is 20.8 mm above the groove section so a good clearance from the 22.14 diameter piston.

I hope I now have a solution. Pic shows B&G seals on left and AP Caparo on the right. Both were well worked into the groove.


John Francis

This thread was discussed between 27/06/2014 and 11/07/2014

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