Brakes problem - need Guru's input!
 

Hi all!

I got a weak breaks problem
changed almost everything:
new ss brakelines
new Power Slot rotors
new Hawk HPS pads
remanufactured brake booster
overhauled master cylinder

but brakes are still weak and not consistent - one time i push the pedal they're ok, the other they're weak...

so i believe the only thing left to be changed is proportional valve and maybe NEW (not overhauled) master cylinder.

Any thoughts on this would be welcomed!

Thanks!

Before you go jumping into replacing more things do a few simple checks first such as making sure the caliper slides are free and nicely lubricated and that if you are using anti-squeal plates on the pads that they are positioned correctly over the back of the pad and not hanging half off and getting jammed on something else.
Also make sure that you have bled the master properly if you didnt bench bleed it before you installed it.
Another possibility could be that a piston (or pistons) in a caliper have corroded and isnt sliding properly in its bore.
Hope this is helpful. :)

Edit: Also if the overhauled master was dodgy then it would have a consistently bad pedal not just sometimes.

Ohh... I forgot to mention that calipers were rebuilt as well and pistons are fine and not rusty... Brake fluid is new Liqui Moly DOT 5.1

What do you mean by bench bleeding the master before installation?

The brakes are bad all the time, but some times are worse than others :-/ So it might be the master, right?

To bench bleed you put the master cylinder in a vice, fill up the reservoir with brake fluid and push the piston all the way in with a screwdriver (or whatever floats your boat). You then put your fingers over the outlets so that when you release the piston it pulls in brake fluid instead of air. Repeat until brake fluid comes easily out of all the outlets.
As you have already installed it you could pull it off again or you could grab a mate and get him to pump the brake pedal a few times then hold the pedal down while you crack open one of the outlets at the master, when his foot hits the floor lock off the line and get him to pump it up and push the pedal down again and repeat as necessary with all the lines until fluid comes easily out of all of them.
Im doubtful that the proportioning valve is faulty (not impossible though) it sounds more like you still have air in the system.

Could you please describe the symptoms in more detail, like what exactly happens when they're "weak". Is it a soft pedal, hard pedal, or the pedal feeling normal but the car just not slowing down like it should?

Soft pedal would indicate air in the lines. Hard pedal would indicate a problem with the booster or the vaccum supply. Pedal feeling ok and not slowing would indicate a problem with the pads and rotors most likely.

It's probable that you've gotten a bunch of air into the system with all those changes. Make sure to bleed the rears from both sets of nipples, and try opening them, then pumping a number of times in quick succession, rather than the usual push, open, close, release, repeat. I've done it a few times and it works well at getting those stubborn last air bubbles out. Jack the one end up only and put it as high as you can safely get it to make it easier for the air to get to the caliper.

Thanks for your input guys! Much appreciated!

The pedal is soft all the time and it feels more or less the same, it just sometimes car doesn't stop as it use to last time :(

We've bleeded the system, but maybe not as thorough as we should...

Will let you know if it works!

A member posted that there was a clog in the booster/vac line that caused this. Like with my s4 the brakes work fine, but sometimes they are very strong. Ya know, just tap the pedal and you can just feel them digging in.

But as long as its not hurting anything, I wont go disconnecting hoses and maybe hurt something when all it takes is alittle more weight to get the car stopped.

Yep, sounds like air in the system, which given all the replacement parts isn't surprising. If you can pump the pedal untill they get stiff then it's definetely air in the system. A plugged vac line will lead to a stiff pedal. Boosters can cause an erratic pedal, with it being soft and then hard at times, but it doesn't sound like that's your problem.

a failed booster vac line can also cause the soft pedal syndrome.

How bout this for a brake problem?

When the car is off, the brakes are hard as a rock. When the car is on, the brakes are spongy as hell, until midway through the pedal. What would you consider that?

That sounds like the booster for sure.

*scrounging around to find another booster*

I have THAT exact problem Roen has...
JOHNSON!, NEW BOOSTER stat!