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TR6 Help needed with TR6 Spongy brakes [sometimes]

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Largo_Larry

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I am the proud owner of a 1974 TR6 that is in excellent condition. I do however, have one problem, a spongy brake peddle.

Within the past 500 miles I have replaced all four wheel cylinders, pads and shoes & master brake cylinder. I use silicone fluid and have blend the brakes. There are no signs of leaks and the fluid reservoir level stays full and there are no signs of bubbles. The brakes on all four wheels seem to work evenly and smoothly. The car stops without pulling to one side or the other and they seem to stop the car as well as I might expect from a 1974 vehicle.

However, sometimes the brake peddle feels firm when depressed but other times (even during the same trip) the peddle is spongy (soft) and feels that it will drift all the way to the floor when depressed unless I keep it firm by pumping the peddle. Needless to say, this problem is a little troubling.

Since there are no indication of a leak and all 4 wheel and the master cylinders have been recently replaced (Moss parts), the source of the problem is a little confusing to me. My best guess is that the tipping valve, in the MC, may not be seating properly every time, but that is just a guess.

Before I try rebuilding the master brake cylinder or go to the expense of replacing it again I thought that I would get some advice from other enthusiasts.


What do you think?



Thanks in advance!!!



Larry
 
Try replacing the o-rings in the PDWA. And what do you mean "all 4 wheel cylinders have been replaced"?
 
The brake hoses were replaced about 10 years ago when the car was restored. How can I tell if hoses are the source of the intermitant soft brake peddle?

Last year I found one of the rear brake cylinders leaking so I replaced both rear cylinders with new ones and put on new shoes/pads all the way around. I was wrong about replacing all 4 cylinders, the front calipers were in good shape.

Larry
 
An intermittent problem is a tough one... So there is no fluid loss? You did right replacing the rear wheel cylinders - rebuilding those can be an exercise in futility. If the hoses are good the only things I can think of is the PDWA and the vacuum booster. The booster could have fluid in it, but that shouldn't effect the pedal height, just the ease of application. And if you've replaced the MC I'd rule that out.
 
Just to play devils advocate.... are you certain that ALL the air has been evacuated from the system? Maybe go back and bleed them again to be certain.
 
I'll try bleeding them again. It's worth another try.

What seems odd to me is that the peddle is firm sometimes and then soft enough to slowly go to the floor (like a leak) a few minutes later. However, the fluid level does not change.
 
Largo_Larry said:
What seems odd to me is that the peddle is firm sometimes and then soft enough to slowly go to the floor (like a leak) a few minutes later. However, the fluid level does not change.
Click to expand...
Sure sounds like an MC problem to me.

One of the Stags had the same complaint when I brought it home ... it suffered total hydraulic failure not too long afterwards. Not the same MC as a TR6, but the cup lips were actually torn away from the flat portion of the seal. Apparently sometimes the plunger would close the gap, and sometimes it wouldn't.
 
Larry
I have almost the same problem. I have replaced my Master cylinder with one resleeved from Apple Hydraulics. I put on new front brake lines. Rebuilt the calipers, rebuilt the PDWA, replace both rear wheel cylinders, new rear drums,new front discs, had the vacuum booster rebuilt, new front wheel bearings, tried every way of bleeding with and without vacuum bleeder, put grease around bleeder screws to control air around threads when vacuum bleeding and I still get the same problem.
No leaks, no drop of fluid level in the M/C reservoir, but you get in the care one time and the brake seems soft and another time solid right up top.
I have changed the M/C twice, the first with a unit from R/F (nothing wrong,I could see but with todays parts?) and the next time with the re-sleeved on from Apple.
Sometimes it seems to happen after backing up, and others like you say same trip.
It never acts like it's going to the floor, just soft nor really like pumping pumps it up, it's just different.
I have been stumped but have not given up
 
Larry,

My thoughts are similar to Randall's that it sounds more like a master cylinder issue if the pedal is going to the floor sometimes (slowly) with no loss of fluid. Sounds like either fluid leaking past the seal between the front and rear brake chambers or leaking past whatever seals or blocks off the fluid reservoir.

Sorry, I'm more familiar with the single brake circuit design of the TR4A and not the dual brake circuit of the TR6.

Scott
 
i am not familiar with tr6 brakes, but could it be a faulty servo?
on any car with power brakes, when you have no vacuum supply, you have a spongy pedal.

once again, i'm not familiar with tr6 brakes, but what is the purpose of the "servo non-return valve", could that be the problem?
 
Larry -- are you saying that this problem has persisted even though you have replaced the master cylinder, rear cylinders and brake fluid, or are you saying that this problem only developed recently, after all those things had already been done?

Scott
 
Minesweeper said:
Larry -- are you saying that this problem has persisted even though you have replaced the master cylinder, rear cylinders and brake fluid, or are you saying that this problem only developed recently, after all those things had already been done?

Scott
Click to expand...


Thanks guys for allyour thoughts and experiences.

Scott, good question. Prior to the repairs I recall the peddle being firm but I was losing fluid due to bad seals in the rear wheel cylinders. The current problem has popped up after the repairs but not right afterwards.

I am surprised to hear that there be others who have experienced this same problem even after numerous repairs.

Larry
 
HerronScott said:
Larry,

My thoughts are similar to Randall's that it sounds more like a master cylinder issue if the pedal is going to the floor sometimes (slowly) with no loss of fluid. Sounds like either fluid leaking past the seal between the front and rear brake chambers or leaking past whatever seals or blocks off the fluid reservoir.

Sorry, I'm more familiar with the single brake circuit design of the TR4A and not the dual brake circuit of the TR6.

Scott
Click to expand...

Scott, I'm still thinking that the problem maybe in the MC even though I replaced it. I'm not sure how hard it is to rebuild it. If bleeding the brakes again doesn't work then I may go ahead and pull the MC apart. What the heck that's why we own these cars isn't it, working on them is half the fun.

Larry
 
Largo_Larry said:
Scott, I'm still thinking that the problem maybe in the MC even though I replaced it. I'm not sure how hard it is to rebuild it. If bleeding the brakes again doesn't work then I may go ahead and pull the MC apart. What the heck that's why we own these cars isn't it, working on them is half the fun.

Larry
Click to expand...

I agree with the others that it does sound like a master cylinder problem. I have heard several reports of the new master cylinders going bad fairly soon after installation (i.e., weeks or months). I also had a problem with an Apple Hydraulics rebuilt master -- brakes would work fine, but then would go to the floor on the next application and require pumping to get pressure. I eventually determined that the master cylinder piston was hanging-up. By pumping the pedal, I wasn't actually building up pressure, but nudging the piston until it finally sprang back into position. Apple sent another rebuilt unit an all has been fine since -- nice, firm brakes.

The other Scott
 
OK, I did a little further investigation and here is what I found:

First, upon looking closer I noticed that there was a lot of black specks on the bottom of the resevoir, both front and back sections.

Second, after pulling the MC from the car I noticed that even more black specks came out from both exit ports as I drained the MC into a container (not a lot but some).

Third, the tipping valve seems to be operating freely and appears to seal off properly when the plunger is exercised.

Fourth, When I bench tested the MC with fluid in the resevoir I noticed that I could pump fluid, under pressure, through the rear port but not from the front port. There does not seem to be any pressure being created on the front end.

So, at a minimum it looks like the MC needs to be rebuilt (maybe replaced). However, I'm have trouble trying to get the tipping valve securing nut loose. How much torque should I expect to apply to loosen it? Doesn't it turn CCW to loosen?

Also, I'm trying to make sure that I order the correct parts but I'm not sure which numbers on the MC to use. There is a red "wrist band" around the MC which is a little hard to read but the numbers look like(6882/2 0/7 60/40 34GE)
There are also a number letters/numbers cast into the part itself. They also are a little hard to make out but they look like (G 648/8258 B375).

Do I need to be more careful about ordering parts then just making sure that either the rebuild kit or replacement MC is made for a 1974 TR6? If so, how do I make sure that I get it right the first time.

Any recommendations regarding parts suppliers? I believe I bought this one from Moss but it does not seem to have held up very well.

Thanks for your input!

Larry

Forth
 
Guys
Just to clarify and to not steal the thread.
Larry, in your case would the pedal actually go to the floor? or just be a lower level than when it felt firm?
Also not matter what level it depressed to, would one more actuation make it firm or did it take pumping?
 
As I push the peddle it initially feels firm and the brakes engage. As I hold pressure on the peddle it begins to slowly drop and feels like it will go all the way to the floor. However, after it drops about 3/4 of the way to the floor I will pump it a few times and the peddle comes back up. At that time it may hold or it may start to drop again.

Right now I'm thinking about just going ahead and order a replacement MC. Any suggestions about where to get it from? Also, are there more than one model of MC for the 74 TR6? If so, how do I make sure I'm ordering the correct part?

Thanks,

Larry
 
Largo_Larry said:
As I push the peddle it initially feels firm and the brakes engage. As I hold pressure on the peddle it begins to slowly drop and feels like it will go all the way to the floor. However, after it drops about 3/4 of the way to the floor I will pump it a few times and the peddle comes back up. At that time it may hold or it may start to drop again.
Click to expand...
I had this exact symptom on the GT6's dual master cylinder. Installing a rebuild kit returned things to a firm pedal with no "drop", but inspection of parts never led me to pinpoint or even guess what was the cause.
 
I just checked with British Parts Northwest regarding one of the MC's they offer for my TR6. At least for the one I asked about they said use of silicone fluid would void the warranty and may result in problems.

I thought silicone brake fluid was suitable for this application and an enhancement. Is there a specific model of MC that I should be looking for?

Larry
 
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