the blue one

been a long time since i last posted....7 years goes by quick. I've spent a few hours searching for an answer for this, and although ive seen my symptoms described several times there were no clear answers.

My drivers rear brakeline blew out, so i repaired it and bled it. the brakes still worked, but the top 60% of travel did almost nothing. i could pump it hard, but then it would sink back down. if i stomp it hard and fast enough, i can lock up the front brakes. i would say that the last 40% of travel feels as it should feel at the top. it does become hard at the last 1/4''....rock hard and flexing the fire wall, probably just the end of travel.
Now that its been a few years, I just went and replaced the rear brake lines from the master to the drums, the drum pistons, shoes, hardware...im glad i did because it was time. I then put about 2 tall bottles of brake fluid through trying to bleed them. I bled them with suction, the old fashion way, and gravity. I also went ahead and bled the master, but only in the car. none of this did anything at all to change what the brakes did at all. not even a tiny bit better.
the front bleeders are stripped and shot, and i didnt feel that they were the issue (or the booster) since i can lock them up.

i did already buy a new master, but really would like opinions on whether you guys think im wasting my time and should do a bench bleed on a 36 year old master cylinder. Also if that doesnt work, then what will?

sofakingdom

iTrader: (1)

As cheeeeeeeeeeeeeeeeep as the MC is, I think you already know the answer to this question.

Don't forget to bench bleed the new one, otherwise it'll act just like the old one. In case you haven't figured it out yet, the MC is at such an angle in these cars, that the fittings for the lines (which are the only way any trapped air can get bled out) are BELOW the highest part of the cyl, which means that it will NEVER come out with the MC in its installed position. You can bleed a gallon of fluid through them with vacuum, then another with pressure, then another with ... whatever else, EVERY SINGLE DAY, and even twice on Sundays just for good measure, and that air will STILL be up there laughing at you FOR ALL ETERNITY.

Next, since calipers are STOOOOOOPID CHEEEEEEEEEEEEEEP as well, might as well swap those out at the same time; except this time, don't let the MC run dry.

the blue one

yeah im going to order new calipers today. i just wanted to make sure this seemed right, not too keen on what a bad master cylinder feels/looks like, i never had a bad one before, and im no pro. ide hate to have missed a stupid simple thing and be swapping out parts because of it

tom3

Usually a bad master will either leak or it will slowly leak internally between the two systems. Put on the brakes pretty hard, let off just a bit, then push back down and hold. If it stays firm and isn't leaking it's probably ok but still might need unbolted and bled.

sofakingdom

iTrader: (1)

Except of course, it's 36 yrs old, and he already has a new one.

tom3

If it wasn't bench bled correctly there could be an air bubble trapped at the head the way the MC is mounted and it doesn't take much to give a sponge pedal. I like to use a vacuum pump to bleed brakes when I replace lines and cylinders. Also have rigged up a cover for an iron master cylinder with an air tap to pressurize the whole system and really flush out old fluid and air. Sometimes it's just tough to get it right no matter what.

sofakingdom

iTrader: (1)

A pump or a vacuum won't get the air out of the MC in these cars. The air bubble is ABOVE the ONLY port by which it would be able to escape, as long as the MC is installed to the car. It doesn't matter what pressure you put to it; the best you can do is to compress the air bubble, which then when the pressure is removed, it will simply expand back to its original size, STILL IN PLACE, since there's nowhere for it to go.

For these cars, there is NO SUBSTITUTE for "bench bleeding" the MC, in whatever form that might take, by holding it level and bleeding it fully, BEFORE final installation. By doing this, all of the "tough to get it right" is MAGICALLY put to an end.

Then AFTER THAT, any of the various vacuum and pressure methods and tools work successfully on THE REST of the system. Butt for the MC itself, no way.

In this case, the OP's car popped a rear line. This would have drained the MC more or less instantly, introducing air into the part of it that CAN'T be bled while it's on the car. Which is why his brakes have NEVER worked right ever since that, no matter how many times or by what method he has "bled" them. The ONLY solution is to "bench bleed" the MC. Which since it has to come completely off ANYWAY, and he already has a new one ANYWAY, it makes no sense to do all that work to the OE one, when he can perform the EXACT SAME labor to the new one he ALREADY HAS. Do all the EXACT SAME labor, just, set aside the old one and pick up the new one in between the "remove" step and the "bleed" step. Might even be LESS labor since the new one won't be all rusty and full of contamination and covered with the funk of 10,000 years.

Calipers are like $25 - 30 apiece for these cars. DIRT cheeeeeeeep. Might as well do those at the same time, ESPECIALLY if the bleeders are destroyed, which makes bleeding THEM a problem too. Possible butt hard to be sure it's complete. They're however many years old too; full of rust and in whatever other kind of bad shape may have accumulated over all that time; time to renew them and make the whole system correct and safe.