Oheyo--

Got an interesting issue. On my '81 turbo tercel, after a few "hard" tuning passes (12psi tops), I notice that my brakes are going spungy, and the rotors are the temperature of the sun (boiling brake fluid, using dot 4). With this being an old project car, I thought maybe its a dragging caliper; however, these are know good calipers, the rubber lines are new.

I was wondering, if you positively pressurize a brake booster, could it in theory put pressure on the brake master, causing the brakes to drag? My tuning passes are close to home, and when the pedal goes soft I drive it home and quickly jack it up, only to find the wheels spin very easily. My booster does have a check valve, though with it being 42 years old, I'm wondering if its allowing a small amount of boost through to the booster.

Thoughts?

 

I always thought it work opposite it should give you a hard brake pedal when lacking vacuum. The check valve should be cheap enough to give it a shot.

 

Brake fluids get hot and air inside them expands leading to a spongy feel. New parts might have air trapped from previous replacement, I would bleed them, then, replace the check valve if you can get one just as general maintenance but KEEP THE OLD ONE just in case sometimes new is worse than old. Also the lines could get hot and expand if they are cheaply made I guess. Consider braided brake hoses from DOT compliance.

 

a quick check would be to remove vac hose and plug it off, then go for a drive in manual brake mode and see if it repeats. that will isolate if it is a boost leaking into the master or not.

boosters under pressure instead of vac will be hard( first hand experience) then go back to normal once manifold vac returns. the problem i think you have if it is boost related is that the booster seals to the master and the master has a cup seal, they only really seal pressure well in one direction and manifold pressure might be getting into the brake system. although 10 psi should not be applying shit to the brakes.

 

Why there is not a check valve in the brake booster?

 

I had a customer who put his brake booster check valve in backwards, the booster would only get positive pressure, the pedal would get so stiff you could bend the brake pedal arm before engaging the brakes.

Pads can also drag without seized calipers holding them, I always file the brake pad mount tabs (that slide in the carrier) and try them backward (metal first) in the carrier to make sure they wont jam over time with pad wear.

You are probably over taxing the 81 Tercel brake system, try some portefield street pads.

 

Its external.

I just went through and rebuilt the calipers and rebled all 4 corners. It helped, but the brakes still feel like ass. The fronts are still getting very hot, but I found out the smoke is from drops of oil somehow getting on the hot side of the turbo. I have an identical tercel that had similar brake problems where the pedal felt awful and wouldnt stop well. I ended up changing the booster for a newer generation tercel one and put a known good master on and all issues went away. Prolly need to do the same with this car.

 

Might have to flush the whole system unless it’s full of fresh brand new fluid. Could upgrade to dot 5.1 fluid(not dot 5) for a higher boiling point may not be but should be an improvement.does any of the hotside or exhaust run near any of the brake lines? Might need heat protection. Maybe the proportioning valve is causing the problem. If the pedal is rock hard you’d have a vacuum problem. Sounds more like a hydraulic problem

 

exhaust kinda does, but its not like 2 inches away or anything. the closest part i wrapped in aluminum duct tape as thats all i really had, though its prolly not going to help. i flushed it all for dot 4, and if it keeps boiling after doing proper heat protection, ill jump to 5.1. This generation of tercel doesnt use a p-valve. brake lines are ran in an x configuration -- FR+RL, FL+RR are tied together

 

heat wrap the exhaust and the brake lines where they come near each other.
So no prop valve huh. There isn’t much left to the system then other than the master cylinder maybe fluid is bypassing . Just thinking out loud.

 

i really dont want to wrap it unless i have to. ive done it once and it rotted the pipes out underneath the wrap.

ive had a bad booster imitate a bad master before. I'm trying to find a booster from a third gen tercel to install, but finding them used is kinda tricky.