Poor brake performance
 

Confession: My brakes suck

I have a 95 850 wagon NA. The brakes have been weak since I bought the car. Its most noticeable on the highway. They lack any grab. And sometimes, especially when its raining out, it feels like they do not work on the highway.

I have replaced the front pads. The rear pads still have plenty of material left. Nothing is seized in the calipers as far as I can tell. Recently I tried bleeding the system and flushed a 32 oz container of dot 3/4 fluid through the system. This did not make any difference.

If I give the brake pedal a single pump first, the brakes become much more responsive.

My hunch is the master cylinder. But I wanted to get some other thoughts before I replaced it. I very much dislike replacing MC's.

Did you bleed the brakes in the right order? Goes from BR, BL, FR, FL.

If so then yes I would start suspecting the master cylinder.

If you had no problem pushing the pistons in when removing the calipers and putting new pads on then they are fine.

Why does the order of which corner you bleed matter?

I won't bleed brakes w/o a pwr bleeder any more; so much easier and you won't get air in the line.

Maybe it's the brake booster that's sucking wind?

Have you tested the booster? Without the engine running, pump the pedal a couple of times, then step on it as hard as you can and hold it. Now start the engine. If booster is working, pedal will drop another half inch or so.

If it doesn't, either booster diaphragm is leaking, or the vacuum line to it is (also an inline ck valve that could be broken). If all that cks out, then yeah, maybe the MC is due...

Booster will make it hard to apply but the actual friction and stopping wouldn't be different just the amount of effort (second foot) to apply.

The sequence can make a difference. Some of the GM mid platforms in the 80's actually needed to be jacked up in the rear when doing a master because the master was angled up so heavily. Try to bleed it flat on the ground and you could send gallons through with pressure or not and still have air.

It's too late to think (1am) that hard but in some systems you have to follow a sequence or air just migrates from one line to another and back and never moves through the system. Could also be soft rubber lines. Mine were badly weathered and cracked when I bought mine. The pressure that's supposed to be clamping the pads to the rotor is actually just expanding the hose(s).

Did you just do a pad slap or did you surface the rotors? A pad slap can actually leave you with less brake efficiency because you have new flat pads riding on a used uneven rotor so the surface contact area is greatly diminished.

I would also give that master a good look. If a quick pump before you actually apply them helps it's either air in the system or a bad master. Could be one of the rubbers inside is actually bypassing some of the fluid/pressure so it's not all getting out into the system. Doing that little pump wouldn't help a leaking booster work better. In fact it would bleed off some of the vacuum that it uses to assist in application.

Thanks for the comments.

-I don't have any indication that the vacuum booster is faulty.

-The car is 18 years old now. The flexible brake lines could be flexing. I'll take a look and see if there is any extensive flexing when applying the brakes.

-I bled the caliper furthest from the master cylinder first and closest last. So right rear first. I pulled over 8 oz of fluid through that caliper.

-I don't have a pressure bleeder, I wish I did. I just had to do it the old fashion way with a friend pumping the brakes. Its a tried and true method.

-I can't remember if I did a pad slap on the front brakes. This was at least a year ago. But I do remember braking performance didn't change after the repair.