TR6 TR6 brake issues

[Phil73]

Having a problem with the brakes on my TR6 hoping someone here can shed some light.

Last year I had a lot of trouble with the brakes on the car pulling to the right under hard braking, leaking from the calipers and other various issues. At the end of the driving season I vowed to exercise my brake demons once and for all before putting the car back on the road this spring.

Over the past month I have done the following:
- Replaced the front calipers with new calipers
- Rebuilt the rear wheel cylinders
- Replaced all steel brake lines
- Put new seals in the master cylinder (MC had been sleeved years ago).
- Had the brake servo rebuilt by a professional
- Flexible lines are only 2 years old
- Bench bled the MC
- Filled the system with DOT 5 and bled the system multiple times to make certain no air bubbles are present.

In short, I rebuilt or replaced everything.

Now, with everything theoretically ready to go I have an unexpected problem. The pedal is nice and firm when I put my foot on it when the car isn't running. However, if I keep pressure on the pedal and the start the car the pedal will slowly sink to the floor. Pumping the pedal gives some pressure back but if I keep my foot on the pedal it will once again slowly sink to the floor. Driving the car around the block I found that the brakes are so weak that they barely stop the car from 20 miles an hour. Locking up the brakes is out of the question.

I suspect the brake servo is malfunctioning but can't be sure and would find that surprising given that it was just rebuilt.

Any ideas on what problem I'm chasing here?

 

[TexasKnucklehead]

Is there an adjustment on the rod between the servo and the MC?

 

[TR3driver]

My first guess would be that the MC is leaking internally, or perhaps not assembled properly.

Does the level in the reservoir drop if you let the pedal sink to the floor a few times? If not, then the fluid from inside the MC almost has to be leaking back into the reservoir instead of somewhere else.

 

[HerronScott]

I agree with Randall.

Scott

 

[Phil73]

Thanks for the responses. If the MC were malfunctioning, wouldn't I see the same problem even when the car is off and the brake booster is inactive?

 

[Phil73]

Update: Based on the feedback here I tried an experiment this morning. Went out and put my foot on the pedal when the car wasn't running and pressed HARD. It took a while but the pedal did eventually go to the floor. It just did it VERY slowly and under HEAVY pressure.

That would imply that my freshly rebuilt brake servo is, in fact, working fine and the MC is malfunctioning...right?

Can't figure what would be wrong with the MC though. Its not leaking, all the seals are brand new and the cylinder bore was sleeved with brass a number of years ago. It was working fine in the fall.

Is it possible that the switch to DOT 5 is allowing it to not keep pressure?

I'm at a bit of a loss here but the first obvious step is to pull the blasted thing out again and take a look.

 

[M_Pied_Lourd]

Phil,

How is the condition of the vaccuum line between the booster and the manifold? Is it possible that it is collapsing internally?

Cheers,
Tush

 

[Phil73]

Line from the manifold and booster is only a couple years old but I'll take a look at it.

 

[DrEntropy]

And at renewing the MC seals did you run a stone LIGHTLY thru the bore first?

 

[TFB]

I think checking the seal of the non return valve in the master cylinder might eliminate one possibility.
Did you also replace that seal?Not sure the easiest way to check when installed and filled.Easy to check on bench.I guess you could try and disconnect the resorvoir and see if fluid flows out the res.port under pedal pressue.
tom

 

[Phil73]

Took out the MC and PWDA, disassembled both and took a look at everything. Nothing seems amiss.

- Shuttle in the PWDA is where it belong and allows no leakage between the 2 sides (tested with compressed air)
- All the seals in the MC look fine and new just like they should
- Tipping valve works fine
- No foreign materials in the cylinder bore.
- All ports are clear

I put the MC back together and hooked up a little arrangement that allows me to apply compressed air to the front or back side of the MC. Did the following:

- Applied air to the front port of the MC. Air, of course, came out the top where the reservoir would be.
- Pushed the piston in with a screwdriver so the tipping valve closed
- No air leaks up to 30 PSI
- Switched my air supply to the rear port. Air, again, came out the top where the reservoir would be.
- Pushed the piston in with a screwdriver so the tipping valve closed and the rear seal closed
- No air leaks up to 30 PSI

That would seem to imply that the MC is functioning properly but I haven't fixed anything yet so I'm hesitant to put things back together. and waste another $23 quart of DOT 5.

Any suggestions on other things to look at?

 

[SkinnedKnuckles]

I'm scratching my head, Phil. I don't know what pressures are in the bore when the brakes are pressed, but maybe you need to put it back together.

 

[Phil73]

Had an epiphany last night while I slept. Thought of one last thing I hadn't checked. There's a little spring washer that sits behind the little plunger that seals off the hole in the end of the cylinder so that fluid doesn't back up into the reservoir. Its DL74 in this pic: https://www.zeni.net/trf/TR6bluebook/120.php. I woke up this morning realizing that I don't recall putting a new one in when I did the rebuild. Went out and took that portion of the MC apart and sure enough its not there.

Seems like a trivial little part and I'm not certain what purpose it serves but I have to wonder if that's the source of my problem.

Now, unfortunately, I don't have one. My rebuild kit didn't come with one or I'd have put it in fpr sure. Think I could pick up such a thing at a decent hardware store or should I just order a TRW MC rebuild kit from TRF and call it done?

 

[TFB]

Suprised you passed the air test without that spring.When you start to press the plunger the rod releases the spring to press on the sealand close resevoir port..
You could have my old ones but suspect TR3 might be different.
Tom