My brakes are broken - pedal goes soft when booster connected

[86coupe]

Car - JDM ae86 coupe with 4 wheel discs

motor - 4age

use - transportation device

problem - when engine turned on, brake pedal goes soft while booster is connected

occurs/started when - noticed at last track day, pedal was moving too far down before the brakes started biting. I've just rebuilt the front calipers, and now its so bad I cannot drive the car.

Have bled all the brakes multiple times, and flushed two complete bottles of fluid through the system now. Pedal feels normal with booster disconnected, but once connected, the pedal has virtually no resistance until the last cm of travel. Initially suspected faulty master cylinder, so replaced that with one off another car in unknown condition. Problem remains.

Confused in Cairns.

[blake.]

replace booster? i didnt give that much diagnosis but its what i'd do.

[86coupe]

That is the next step. But at present, I don't have a spare booster. And, from what I've seen, a failed brake booster makes the pedal harder to press?

[blake.]

sounds right but you never know aye. any leaks in the brakes or booster?

[Hen may possibly be a nut]

It sounds like classic air in the lines syptoms. Check that the front calipers are on the right way up (bleeder at the top).

[86coupe]

I didn't realise they could be installed wrong, but yes, they are up the right way. I've put 3 bottles of fluid through it today with no effect. Where is this air hiding? Have had the calipers off a few times in the past and not had any problems bleeding the air out afterwards.

[Sam-Q]

maybe your brake master internal seal is stuffed?

[biggo]

Yair, diapham has a hole in it or theres a pad that goes between the push rod of the master and diapham that can fall out in the booster. best bet is score a 2nd hand one.

with the engine not running, can you get a firm pedal?

[86coupe]

Yeah, not sure what condition that master cylinder is in. We bled more air out of it yesterday. Felt reasonable, but after a few minutes of driving, pedal was soft and useless again. Certainly wasn't good enough to take to the track, so will drop it off at a brake place next week and they can deal with it. One stupid thing I did in my rush to get the thing fixed, was use an unsuitable lubricant on the seals. I pulled the calipers apart again and replaced the seals using a proper caliper assembly lube.

[86TRD]

maybe your brake master internal seal is stuffed?

if the seal is stuffed it should leak fluid shouldnt it? or not always?

[Sam-Q]

no not all all, it can be bypassing internally