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MG MGA - Master cylinder rebuild problem

I've just been rebuilding the master cylinder - I have honed out the cylinders, cleaned everything spotlessly, and put it back together with a repair kit from Lockheed.

The problem is, having replaced all the rubber washers in the right place, the pistons are now jammed in the cylinders - both clutch and brake. Having refitted them, and started to bleed the system, I press the clutch pedal - all this does is jam the piston further down the cylinder. It just won't move.

Any idea why this should be happening with standard parts fitted (the pistons are the original spec, as is the cylinder itself), and what I can do to get the pistons back out??

Any thoughts most welcome.
Richard Ross



Richard,

Compresed air will get the pistons back out but be carefull, wear eye protection from the spray (cover any painted surfaces etc.)

I had a similar problem with my rebuild and it was because I installed the rubber in the wrong place. No clear instructions came with mine and I labeled everything but some parts had been torn up very badly. I remember I was able to put one of the rebuild kits pieces over the metal slide in the wrong way, (it's been a while)Same symptoms as yours. The rubber appears to fit a certain way but there was another way to make it work, not so obviuos that I eventually figured out. The diagrams I had did not help as they did not show what lips get covered etc.

It was trial and error for me, sorry I couldn't be more specific.

Good luck
Tom M

Richard,

I agree with Tom on the compressed air idea. I ran into a similar problem. Stock MC, old parts moved fine just leaked a bit.

Took very good notes and even some digital pics to ensure it would go back the right way.

Well both pistons got stuck as did yours. I couldn't figure it out for the longest time, then I checked the parts themselves and found the cups to be too small in diameter. Not by much, but enough to bind the pistons. Let me re-phrase, the outside diameter of the cup was the same, it was the INSIDE diameter, you know where the seal rides on the piston was smaller. When installed, it made the outside diameter of the cup too big for the bores and binding occured.

Luckily, and I mean luckily, I had another old kit that I was afraid to use (because of age) used it and all is well a year later. Hope this gives you a bit of direction.

Tom
Tom Baker

Thanks for the tips ...

dropped into my local garage on the way into work this morning ...

compressed air certainly got the pistons out (although one came out a little more energetically than we had planned).

Mechanic confirmed I'd got the washers the correct way around - but told me I should have used rubber grease on all the washers, and a light smear inside the cylinder bore too. He assured me this is probably causing the problem.

I suspect that Tom (Tokyo Tom that is) may be on to something with his diagnosis ... as the pistons came out, the main washer had come off the collar on the piston and slipped slightly down the shaft.

Has anyone else experienced similar problems with Lockheed repair parts?
Richard Ross

A couple of comments regarding tight pistons...
As has been discussed previously on this BBS, there seems to be a supply of piston seals out there that do not fit properly. I have suspected (only a guess) that suppliers may make seals a tiny bit oversize to compensate for the enlargement that occurs to bores over years of use and honing, thus hedging the bet against leakage when new seals are fitted to old units. I had a situation where, from the same kit, one seal was OK and the other was tight, even when bores were swapped.
Having said that, there are at least two different styles of piston bodies that I know of. One is made from aluminum and one is made from steel. They have different designs in the area where the seal fits. I've had better success fitting new seals to the aluminum bodies then to the steel ones. Once upon a time, when you bought a brake kit, you got the whole piston, with seals fitted and new springs. Also, springs can go weak over time. May be a good idea to fit new ones.
Regarding grease, while it may help to use the special brake grease, its hard to find and it really should not be needed. Lubrication with brake fluid should be all that is necessary.

Regards,
GTF
G T Foster

One other alternative to the sticking piston: At least one supplier of rebuilt M/C's used to slightly overbore the cylinders, and replace the pistons with ones made for the increased bore. Presumably the original seal kit will not work on these. Perhaps Richard has one of these rebuilt M/C's.

I have one of these on my car. The supplier I bought it from no longer supplies the M/C's rebuilt in this manner. I don't know where to get a rebuild kit for these. When I have to rebuild it (hopefully many years from now), I will have to bore and sleeve the M/C and find some old pistons.
Chuck Schaefer

Having just looked back through the archive, I realise this is a subject that's been much discussed already.
Thanks for bearing with me.
For some reason Moss inadvertently sent me two rebuild kits instead of one, so i may have to compare the diameter of the washers.

The up-side is that I could be a bit more thorough with my honing ... I had worried that I was taking too much off the sides ever to achieve a tight enough fit ... how wrong I was.

Richard Ross

Hi Chuck. You mention "MGA" master cylinders with 2 different bore sizes, the bigger of which you say was bored oversize by a rebuilder. That may or may not be correct. Austin Healey bug eye sprites used a master cylinder that externally looked identical to the MGA master cylinder. It housed both brake and clutch cylinders in the same iron casting the MGA's used. However, internally, the bug eye master cylinder bores were a different size than the MGA's. (I dont recall if they were larger or smaller) I know this because many years ago, I went to rebuild what I thought was an MGA master cylinder, and the rebuild kit parts WOULD NOT fit properly. After talking with many British car people, one old fellow told me I might have a bug eye sprite master cylinder. When I ordered bug eye master cylinder parts, they fit and worked perfectly. Just thought I should mention this possibility, for anyone interested. Cheers! Glenn
Glenn

Glenn, I am aware that there are differences with the Bugeye vs MGA. This is yet one more possibility for a misfit of replacement parts in restoring an MGA. If I remember correctly, the MGA is a 7/8" diabore and the bugeye is 3/4" dia.

The fellow that rebuilt the M/C oversize was (I think) Eric Jones out of Ohio? He always shows up at the British Car Swap meet here in the Chicago area in February. He had both the bugeye and the MGA M/C's. He rebuilds (or resells)heads, generators, master cylinders etc. (stuff that takes machining). I bought my M/C from him about 6 years ago and returned the core to him the following year. At that time, he said he was no longer associated with the fellow who rebuilt them oversize.

Chuck Schaefer

Hi Chuck. I stand corrected. Thanks for the information! It is interesting to discover things about the parts we encounter when working on our cars. I guess nothing should surprise us, given the age of the cars, and the parts! Cheers. Glenn
Glenn

Just to give you the conclusion to my attempts ... I made a simple error, which had a simple solution.

I simply didn't lubricate the rubbers, which is clear to me now as the only reason why they jammed - but they jammed solid.

I refitted the pistons (with exactly the same rubber whashers) after smearing the inside of the bore AND the outside of the rubber washers with rubber grease/ hydraulic grease, and now it all moves perfectly. I'm sure simply using brake fluid might have worked just as well.

It seems obvious now, but none of the literature I've read ever mentioned lubricating it all before reassembly. You live and learn.
Richard Ross

Gentlemen: We spent a lot of time investigating complaints of MGA master cylinder kits not fitting correctly. Here is the gist of what we discovered:
In the US there were two sources of overbored MGA cylinders during the 80s. Roadstar Nelson and Beck Arnley both manufactured their own pistons and seals to fit overbored cylinders. This problem is prevelant enough that a major rebuilding company has manufactured their own replacement pistons as too many units are coming through with either incorrect pistons or ones too badly corroded for reuse.

In addition we have followed a number of threads, such as this one which have resolved when correct brake lube was used, or weak defective springs replaced. So far this year we have sold over 600 kits, with only 7 returns over the last 3 years. The kit is pretty cheap, so it is possible that the return rate does not accurately reflect customer satisfaction. But until there is any strong evidence to the contrary we are comfortable stating that when installed correctly the Lockheed MGA brake/clutch rebuild kit is currently correct.

Safety Faster

Kelvin.

KJ Dodd

This thread was discussed between 24/10/2004 and 29/10/2004

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