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TR6 Brake Pressure Problem

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SherpaPilot

SherpaPilot

Jedi Hopeful
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1972 TR6, stock brakes: I recently flushed my brake system and replenished it with Castrol Dot 4 fluid. Each corner was properly bled with great results and good pedal pressure. After installing each wheel and putting the car back on the ground, I started the engine and warmed the car up. With the engine running at normal idle rpm, my brake pressure seemed to drop considerably. After shutting the engine down, my brake pressure returned to normal pressure. So, I am now thinking something is wrong with the booster. All vacuum lines are new with no obvious leaks. I believe this drop in pressure is due to a faulty booster but it doesn't make any sense to me. One would think that the loss of the booster would make the pedal more difficult to depress and not the opposite.

Does anyone have any experience with this happening? I am not sure what my next move will be with this brake system. Could this be vacuum related???? Maybe I need to plug the vacuum line and see if there is any change without the booster..
 
If by "brake pressure seemed to drop" you mean that it becomes easier to push the pedal, that's what the booster is supposed to do. If the pedal does stop at some reasonable point above the floor and holds there, I'd say you are good enough to test drive at least and you may find that it will stop fine.
Tom
 
What's normal is "feeling" the brake pedal drop a bit when the engine first starts up and after a pump or two "feeling" the pedal rise a bit when the engine dies..
"feeling" meaning resting, not pressing your foot on the brake pedal.
If you don't feel those movements, that's usually an indication of a problem with the servo.
 
Hey guys thanks for the responses. I have owned this car for about 30 years and had always thought the brakes could be a bit better. The reason I dove into this brake issue was because I flushed my brake system and re-serviced with Castrol. The bleed process went well (did it twice). It just seemed to me the pedal distance (from static to floorboard) decreased when the booster was energized with vacuum just like Poolboy stated. But it also seemed that brake effectiveness did not occur until the pedal was pretty close to the floorboard. With the engine off and obviously no vacuum, it felt more like normal. I don't know, maybe its just my imagination but then again, this booster is the original from 1972. Maybe I just need to replace the booster. I wish Triumph had eliminated the power boost system and designed a simple dual master cylinder instead. If I had extra cash, I would upgrade the system, but other things are more pressing. Thanks again for your comments, they are appreciated.
 
Strange symptoms.....Firstly, it would be interesting to see what the outcome is by disconnecting the vacuum line to the booster.
Another consideration is the length of the push rod connecting the booster to M/C.
 
malbaby said:
Another consideration is the length of the push rod connecting the booster to M/C.
Click to expand...
Good thought and I think that is adjustable.
Tom
 
pdplot said:
On our Honda, the brake pedal goes down when you start the car. It's normal.
Click to expand...

Yes, I tried to indicate that in my initial reply but there is a limit that the pedal should safely "go down" when vacuum is applied. From what SherpaPilot seems to be saying is that his pedal goes beyond that limit "pretty close to the floorboard".
Tom
 
Must be a master cylinder rubber seal problem, it is not booster related.
Fit a new M/C or have yours reconditioned.
 
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