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MG TD TF 1500 - Sticking Brake Master Cylinder
|I just had a m/c re-sleeved in stainless (it measures correctly), installing a genuine Lockheed brake kit and a new return spring. But now I find that the piston does not return easily or reliably. I have tried three different 7/8" cup washers to no improvement, but don't have any extra piston seals to try. I'll order some, and see if that improves matters.|
Any other thoughts why this isn't working - it's a new one for me! I don't have any, but would brake grease help? I lubricated the parts with brake fluid, but in bench-testing the piston sticks however far down I pushed it.
|Tom - You might want to check the finish of the bore. Stainless steel is not an easy material to machine and often winds up with a rough surface (or maybe I should say a less that perfectly polished surface). Cheers - Dave|
|Tom, it sounds as if you're running into the situation that I ran into a couple of years ago. The secondary piston cup is slightly oversize. I'll try to find the thread on it. The ridge on the push-rod that locates the cup doesn't fit into the recess in the cup. It took a number of emails back and forth with the supplier in England to get a cup that worked. Symptom is that you push the rod in fine, but it's hesitant to come back out. George Butz helped me to find the cause. Bud|
|I've forwarded copies of emails from 2011 to you and David. BTW, I meant 'piston, not 'rod'. Here's text to Ken Hyndman at Moss:|
Thanks for getting back to me. The bottom line is that I suspect that the cup washer being provided in the 180-910 master cylinder repair kit is defective.
I've run into something while restoring a 53 TD (http://www.ttalk.info/The53.htm) that may have wider ramifications. The car has a Lockheed master cylinder that may well be the one installed at Abingdon. I'm trying to restore, rather than replace wherever I can. This led me to rebuild the wheel cylinders and master cylinder. Must admit that I do most of my purchasing from Abingdon Spares. I bought their master cylinder rebuild kit (31-019), cleaned and honed the m/c and installed the parts. One of the first steps was to bench bleed the unit, but something was wrong. When I pushed in the master cylinder plunger #20 it didn't come back out under the force of the m/c spring #13. To disassemble the components I had to push in from the back of the m/c. The culprit is the cup washer (#16) that is on the piston (#15).
There is a ridge on the piston that engages a groove in the washer. When the groove is engaged that diameter of the expanded washer is too large and requires more pressure than the spring provides to return the piston to its normal starting position. I've performed tests using two similar master cylinders with two pistons and a number of cup washers. The problem occurs whenever a new cup washer is installed.
From old and new rebuild kits I've determined that Nelson Brovex (now Nelson Stoke) produces these kits. I've contacted them:
Thank you for the response. I may not be the only one who has experienced this problem. Perhaps I'm the first to identify it. A few months ago another TD-TF'er on the MG UKBBS reported symptoms that would match this. There are reports of folks having problems with kits purchased from Moss Motors (USA). The cylinder and piston are originals, probably installed in about 1952. The old seal, that fit fine and would work, was found in a kit labelled Nelson Brovex 50567 CJJH. The kit contains the parts that I replaced when I installed in ca. 1960. The recent 50567 DBAJB (labelled 'Made in England') kit was purchased in this past month from Abingdon Spares. I have never purchased directly from you. BTW, please note that the kit is DBAJB, not DBABJ. I couldn't guess how long Abingdon Spares had this in stock. I understand that, except for the bellows, the same components are contained in kits for TC master cylinders. I suspect that the fear of silicon brake fluid being incompatible with rebuilt, i.e., honed, cylinders has significantly reduced the number of us that rebuild ours. I would be willing try another seal in hopes that the one that I have is an anomaly. I've been led to believe that this particular cup washer is available from other sources of parts for Lockheed master cylinders and have such a search underway. Please send the seal to: (A new seal has been received from them, but it has the same problem)
I suspect that your kits are from the same vendor and that Moss customers would run into the same problems. A 'way around the problem' is to put the cup washer on the piston without engaging the ridge in the piston. In this position the washer does not expand far enough to freeze the piston. However, this limits the volume of fluid that can be delivered. IMHO, that's living dangerously.
|I remember the thread now, and it does sound exactly right.|
What to do?
|Tom, by email I'm sending you a copy of the correspondence about their sending me a cup that worked, their #S7464. Bud|
|I recall the problem was that the pistons have sharp edges at the ridge and new kit rubbers had a corresponding area that was rounded, (or vice versa?). This prevented the cup from seating all the way, as it was too big in diameter and would bind in the bore. Bud and I looked at several vintages of pistons (mostly older ones) and all had the same problem with the new seals. An old original Lockheed kit I had fit perfectly. Tom, since you said you used a Lockheed kit, how old was it? And did you mic the inside of the cylinder bore? George|
|George and all - thank you for your input. The overhaul kit was a good 25 years old; I replaced the cup seal (NAPA), but had no newer piston seal. The bore was just re-done by Mark Frappier in SS, and he checks it with a 7/8" mandrel.|
Bud has given me some contact info, and I will try to get a better piston seal.
This thread was discussed between 01/09/2013 and 02/09/2013
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