TR6 TR6 with dot 5 brake fluid

So I have just filled my brake system with dot 5 silicone brake fluid. This system has all new brake lines and flushed and cleaned fairly new brake cylinders. My issue is this: When I apply the brakes they are hard at the top of the travel but with continued application of force, the pedal will slowly return to the fire wall. Have I done something wrong, like not bleeding properly? Would this indicate a problem with the master cylinder or does the booster have something to do with this or is this what one can expect from dot 5 in this old braking design. I have yet to start the engine after my rebuild so not sure how the booster will effect the braking.

Definitely not normal, even with DOT 5, and not a bleeding issue either. It has to be leaking somewhere, but the leak might be back into the MC reservoir or into the brake booster. Did you check the ground for any puddles? Does the level in the MC reservoir drop at all?

Had a similar experience on Stag #1. Wasn't too bad, so I tried to drive it to a shop to have the MC changed (among other work that was already planned). Bad idea! Both sections of the MC quit suddenly during the drive, leaving me with no hydraulic brakes at all. Tried to continue onwards using just the handbrake, but learned the hard way that the rear brakes won't stop a Stag very many times. By the time I got to the freeway offramp, the rear brakes had faded to uselessness. Fortunately, no one was coming at the bottom of the ramp and I was able to make the turn onto the crossroad without hitting anything. After a short break for clean underwear (figuratively) and to let the brakes cool, I made it to the shop with lots of engine braking and no further incidents.

I later found that both cups in the MC were actually torn, as well as being about the consistency of gummy bears.

I have found a couple of connector leaks which tightening does not seem to help. I guess the flaring is not good. I found a leak in the pressure warning switch so I guess a new switch is in order or perhaps a plug and removal of the switch altogether if that is possible. I also have a small leak coming from the back of the booster. Does that mean that the leak is back into the booster as you suggested? Would that necessitate a MC rebuild kit? This MC was fairly new when I bought the car but has sat empty for three years while I underwent the frame off resto.

ceedub said:

I have found a couple of connector leaks which tightening does not seem to help. I guess the flaring is not good. I found a leak in the pressure warning switch so I guess a new switch is in order or perhaps a plug and removal of the switch altogether if that is possible. I also have a small leak coming from the back of the booster. Does that mean that the leak is back into the booster as you suggested? Would that necessitate a MC rebuild kit? This MC was fairly new when I bought the car but has sat empty for three years while I underwent the frame off resto.

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If you have fluid at the PDWA switch, that's not indicative of a bad switch. It means that the PDWA seals are gone. All but the TR250 and part of the 1969 cars use O-ring seals. As long as you have the O-ring type, it's a relatively easy rebuild. Here's a bit on rebuilding both the brake MC and the PDWA: https://www.buckeyetriumphs.org/technical/Brakes/MCPDWA/MC.htm

Just to help regarding the use of DOT 5 brake fluid. I tried using for the first time with my TR3 about 1.5 years ago and I am very happy with the result. One point to make is that the brake system requiring a major overhaul including some new brake lines, new wheel cylinders, master cylinders and front brake caliber rebuilding.

I wanted to point out that I had the front brake calibers professionally rebuilt. I was not able to remove the caliber pistons as they were frozen.

Just wanted to say the Dot5 fluid does work well.

Steve

ceedub said:

I also have a small leak coming from the back of the booster. Does that mean that the leak is back into the booster as you suggested?

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I would take a careful look at the seals in both the booster and the MC. If either has seen DOT-3 or DOT-4 fluid on the wetted parts, buy the new seals or rebuild kits so the rubber parts will not be contaminated. If the TR6 MC has a tipping valve like other Triumphs I have seen, take a look at its seals as well.

When I put everything back together on may car and filled it up I had similar issues with leaks, one or two were fixed by tightening couplings, I know in at least one case I had to re-do the flare on the line, and had to rebuild the PDWA too. Will echo the comments made about DOT 5 too, it works fine, in fact, back when I ran DOT 3 I remember rebuilding or replacing master cylinders and wheel cylinders on a fairly regular basis, the last 15 years or so that I have run DOT 5 (in three different cars) I don't remember ever having any hydraulic parts wearing out and leaking, at least that I can remember, it certainly happens less that is used to.

I had to replace the clutch master cylinder on the blue Spitfire because the bore was parabolic (?) The bore in the middle of the stroke was wider than the ends and fluid leaked when pushing the pedal. This master cylinder was probably more than forty-five years old. I noticed that the DOT 5 fluid seems to be "thinner" that DOT 3 and I had to tighten the unions.

T.T.

Thank you all for your comments. I am more satisfied that Dot 5 is OK for my car. I have purchased a couple of rebuild kits for the MC's and some brake line to redo a couple of flares. I will probably also see if my servo is aftermarket or not (not sure how you tell) and buy a rebuild kit from Moss if it is not aftermarket.