prelude5blue

Hi everyone
it has been awhile since I was online with you, I hoe everyone is well??
my question is-- I changed my master cylinder due to brakes fading and now no matter what I do I cannot bleed the new master cylinder. Any ideas?? I have tried reverse bleeding (vacuum) I have tried regular bleeding, I have bled the ABS system and still no brake fluid from the bleed screws?? can anyone help??

daxr

Did you bench-bleed the master cylinder? That's the first step. You hold the master in a bench vise, thread special fittings into the line outlets the have hoses running back into the reservoir, and manually pump the cylinder until you get clear fluid with no bubbles. Usually it just takes a few minutes.

Most of the time I can bench bleed a master and install it, then, with a helper applying light pressure on the brake pedal, crack open the brake fittings one at a time you've tighened down at the master just a bit to release another bubble or two.

Usually that's it, if you've been careful, and you're good to go with a firm pedal. If you've "gone deep" and opened up various things, and maybe introduced air into different parts of the system, pressure bleeding with a machine may be the best way to go. But you still need to start with a good bench-bled master.

Oh - and verify that new master fits the vehicle, particularly that the pushrod from the booster isn't too long for it, which would make it partially apply the brakes (closing the return ports inside the master cylinder) when at rest.

prelude5blue

hi thanks for the info--yes I did a bench bleed. I didn't break into the system so there shouldn't be any air.
I looked at the manual for the booster push rod but it seems a little ackward to adjust and it is still the factory setting. any thoughts???

daxr

I haven't had to adjust a booster pushrod for years, usually its a non-issue if you've got the right parts. Most of them are non-adjustable. If in doubt measure the depth of the receiver in the rear piston of the master cylinder of the old master and compare that with the depth in the new one. They should be identical, of course.

Once or twice, in a pinch on uncommon models, I've gotten slightly wrong parts and had to swap the correct rear piston from an old master into a new master that had the wrong set-up, for whatever reason. That's easy to get wrong, as you have to make sure everything else is otherwise identical, but its not hard to do really.

You could just have a defective new piece, it does happen. I've gotten rebuilds where a piston seal was missing, where a return spring was missing, where a piston seal was backwards (pumping fluid into the master instead of out!), where line seats were missing or damaged, and where seals were damaged in assembly; so that's possible as well.