Brake booster issues?

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SirFoxx
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My tercel:: 1981 Toyota Tercel w/ 7age

Brake booster issues?

Post by SirFoxx »

Thought I would ask here.

Picked up an '86 t4wd from a buddy of mine a few months back. When I got the car, the brakes felt fine, no issue. I replaced the pads, shoes, wheel cylinders, rubber lines, etc. Basic brake overhaul. I noticed when I was bleeding the brakes, the stream would be solid then suddenly a bunch of air would come out and never go away. After doing this on all 4, the pedal goes straight to the floor with pedal pressure BUT the brakes seem to work alright. Press it to the floor and the rears will lock up. Almost seems like the booster is over-boosting. I also noticed recently when the car is cold and its cold out, brake performance is pretty bad until it warms up.

I thought the issue was the master cylinder, so I replaced it with a new one and same issue. I made sure to bench bleed it before installation as well. I took it to a friends shop and had them power bleed the brakes, same result.

I am not losing fluid anywhere, as indicated by the brake fluid level not changing. My next thought is the booster failed since the car was sitting outside unused for a number of years. I checked the check valve and it also appears to be working.

Thoughts?
1988 DLX 4wd Tercel Wagon w/ weber (RIP)
1985 4wd Tercel SR5 (RIP)
1986 Base 2dr Hatch (RIP / PARTS)
1986 DLX 4wd Tercel Wagon with ???
1985 DLX 2dr Tercel Hatch w/manual swap
1981 2dr Toyota Tercel w/7age (RIP)
1981 3dr hatch Toyota Tercel w/Turbo 4AGE
1981 3dr hatch Toyota Tercel SR-5
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Petros
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My tercel:: '84 Tercel4wd w/extensive mods
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Re: Brake booster issues?

Post by Petros »

it really sounds like the master is bad. I had this exact same problem with a brand new master cylinder. kept pumping bubbles out the wheel bleeders no matter how much we pumped it. I took it out and bench bled it and also got bubbles, so back to the store it went. the replacement bled out and it pumped up fine. Also, makes sure you have all the air out of it.

a bad caliper or wheel cylinder can allow air to get sucked into the lines, but it is not likely you will get air bubbles showing up at the other bleeders. double check all your flex hoses that they are not cracked and allowing air into the lines. A crack or split with the liner forming a flap can act funny, like a one way valve, it will pump up solid without leaks, but when your foot is off the pedal allow air into the line.

I do not see how a bad booster can cause that, it would either work, or not work so you have no power assist. I do not see how it could allow air into the lines. I have also wondered if a bad balancing or proportioning valve could allow air into it, but again it seems very unlikely, they never really go bad.

Consider the braking system is not that complicated, the master is a hydraulic piston that pressurizes the lines, that pushes the wheel cylinders/caliper pistons out to put pressure on the shoes/pads. Air in the whole system usually means air is getting in at the master cylinder. If you get air in only one corner of the system, likely that wheel cylinder/caliper is allowing air into the system. so think it through and hunt down the problem. BTW, do not assume the new out of the box master cylinder is good, my experience is that half of them are bad out of the box. Bench testing should let you know if it is good or not.

Good luck.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)
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SirFoxx
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My tercel:: 1981 Toyota Tercel w/ 7age

Re: Brake booster issues?

Post by SirFoxx »

Petros wrote: Tue Dec 22, 2020 11:25 pm it really sounds like the master is bad. I had this exact same problem with a brand new master cylinder. kept pumping bubbles out the wheel bleeders no matter how much we pumped it. I took it out and bench bled it and also got bubbles, so back to the store it went. the replacement bled out and it pumped up fine. Also, makes sure you have all the air out of it.


This is what I did and it bled fine. Before I got the new one, I put on an old good spare that I had, bench bled it and it had the same pressure issue. The old master came off a first gen tercel, which has a smaller bore than the 2nd gen tercel. I thought that maybe the booster was too strong for it until I got the new master and it did the same thing.
a bad caliper or wheel cylinder can allow air to get sucked into the lines, but it is not likely you will get air bubbles showing up at the other bleeders. double check all your flex hoses that they are not cracked and allowing air into the lines. A crack or split with the liner forming a flap can act funny, like a one way valve, it will pump up solid without leaks, but when your foot is off the pedal allow air into the line.
I am not sure if this is the case. Both rear cylinders are new and the fronts work fine, aka no brake pull. With the car off, the pedal is hard, but when its on and theres vacuum, I lose ALL pressure. See why I think its the booster?
I do not see how a bad booster can cause that, it would either work, or not work so you have no power assist. I do not see how it could allow air into the lines. I have also wondered if a bad balancing or proportioning valve could allow air into it, but again it seems very unlikely, they never really go bad.
While I agree it is quite unusual for a booster to fail like this, I have seen other threads online where this has happened to people. However, most people have not left the solution on their threads.
Consider the braking system is not that complicated, the master is a hydraulic piston that pressurizes the lines, that pushes the wheel cylinders/caliper pistons out to put pressure on the shoes/pads. Air in the whole system usually means air is getting in at the master cylinder. If you get air in only one corner of the system, likely that wheel cylinder/caliper is allowing air into the system. so think it through and hunt down the problem. BTW, do not assume the new out of the box master cylinder is good, my experience is that half of them are bad out of the box. Bench testing should let you know if it is good or not.

Good luck.
1988 DLX 4wd Tercel Wagon w/ weber (RIP)
1985 4wd Tercel SR5 (RIP)
1986 Base 2dr Hatch (RIP / PARTS)
1986 DLX 4wd Tercel Wagon with ???
1985 DLX 2dr Tercel Hatch w/manual swap
1981 2dr Toyota Tercel w/7age (RIP)
1981 3dr hatch Toyota Tercel w/Turbo 4AGE
1981 3dr hatch Toyota Tercel SR-5
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Mark
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Re: Brake booster issues?

Post by Mark »

I've had a similar issue before where there were constantly tiny bubbles when trying to bleed the brakes after changing the wheel cylinders. I went through about 2 1/2 bottles of fluid, and there was still air. It turned out to be the tapered threads on one of the hoses was just worn enough to let air in on the return stroke, but sealed well enough to not allow fluid to leak out. Maybe some of the hose threads on yours are damaged/loose/cross-threaded.
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SirFoxx
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My tercel:: 1981 Toyota Tercel w/ 7age

Re: Brake booster issues?

Post by SirFoxx »

Mark wrote: Wed Dec 23, 2020 8:48 am I've had a similar issue before where there were constantly tiny bubbles when trying to bleed the brakes after changing the wheel cylinders. I went through about 2 1/2 bottles of fluid, and there was still air. It turned out to be the tapered threads on one of the hoses was just worn enough to let air in on the return stroke, but sealed well enough to not allow fluid to leak out. Maybe some of the hose threads on yours are damaged/loose/cross-threaded.
The pedal is not spongy with the car off. Its nice and firm and doesnt have any fade. But when vacuum is applied to the booster, all pressure disappears, but the brakes still work fine....
1988 DLX 4wd Tercel Wagon w/ weber (RIP)
1985 4wd Tercel SR5 (RIP)
1986 Base 2dr Hatch (RIP / PARTS)
1986 DLX 4wd Tercel Wagon with ???
1985 DLX 2dr Tercel Hatch w/manual swap
1981 2dr Toyota Tercel w/7age (RIP)
1981 3dr hatch Toyota Tercel w/Turbo 4AGE
1981 3dr hatch Toyota Tercel SR-5
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