brakes Weak brakes

[Stevenc22]

For over a year i have been chasing weak brakes. When i apply brakes, the pedal feels firm but i never feel like i get more than 70% braking force.

 

I have replaced master three times, calipers twice, booster once, wheel cylinders/drums/shoes twice and i have pumped multiple gallons of fluid through using a pressure bleeder and manual bleeding, I have SS brake lines and have tried various compounds of brakes pads.

 

When i press the pedal, the first 2/3rds of the pedal throw feel good and the car brakes but the last 1/3rd of the the pedal throw feels like nothing additional happens and at high speeds that last third of bake pedal force is the difference between braking suddenly and just slowing down. I have tried everything. Rears are properly adjusted and handbrake is also adjusted. 

 

Interesting lately i noticed even with a pressure bleeder the rear didnt bleed with out the addition of the pedal+pressure bleeder, not sure what that means. 

 

bottom line is pedal feels good but when you really press the pedal hard, the last amount of braking isnt there when you need it most.

 

Thanks

Steven

 

[xferboy]

possible you have a hard line crushed?

[TobyB]

Hmmm.  

 

Quote

 When i apply brakes, the pedal feels firm but i never feel like i get more than 70% braking force.

This is a really hard thing to quantify.

Can you lock the front wheels on pavement?

If you can't, something is wrong.

From your description, it sounds like you are going 'through the booster': in other words,

you're using all the boost available, and then you get a direct connection to the master when it's done.

Which feels like you 'aren't getting any more brake'...

 

Are your front pads wearing evenly?  If only one circuit's working, they will be worn more on 

one side than the other...

 

t

 

[PaulTWinterton]

The only part that you don't mention replacing are the rotors.  I can only add one other thing I know of that defeats the brakes.  Scored rotors. If they are like corduroy, they won't give you maximum braking power. 

 

 

Edited April 16, 2016 by PaulTWinterton

[Stevenc22]

2 hours ago, TobyB said:

Hmmm.  

 

This is a really hard thing to quantify.

Can you lock the front wheels on pavement?

If you can't, something is wrong.

From your description, it sounds like you are going 'through the booster': in other words,

you're using all the boost available, and then you get a direct connection to the master when it's done.

Which feels like you 'aren't getting any more brake'...

 

Are your front pads wearing evenly?  If only one circuit's working, they will be worn more on 

one side than the other...

 

t

 

Toby what does through the booster mean and how do I fix it?

[Stevenc22]

When I use IE ceramic pads the rotors slip when braking because they aren't getting enough clamping force.

[ray_]

Which pads are in there now? Do the current ones look glazed?

 

 

Someone will chime in on which pads are optimal these days. Project car has whatever Blunt sent and it has better initial bite than the Wilwoods on the other car....

 

Also you did replace the rear flex lines thru the trailing arms too right? Tho I don't think that's the issue...

 

Good luck,

[conkitchen]

[TobyB]

Using up the booster means you're not getting enough friction for your pedal push-

the car should just about lock up when the booster is applying all it can, and then

your panic addition of significant extra force gives you good control over wheel lockup.  If only half the

system is working, that will mess that up, as will using something like MetalBastard pads.

 

I don't know what's a good street pad these days- but I'd try something 'street'  from Porterfield, EBC, or Performance Friction...

 

hth,

 

t

 

[Stevenc22]

This isn't a pad issue. I have multiple pads and switch them out and didnt change anything. It's a clamping force issue.

[JsnPpp]

2 hours ago, Stevenc22 said:

This isn't a pad issue. I have multiple pads and switch them out and didnt change anything. It's a clamping force issue.

 

I also agree on the crushed brake line possibility, as well as the rotor and drum glazing.  The fact that you can't bleed the rears w/o pedal AND power bleeder is a concern.  Could be clogged too somewhere in the line.  

 

But do rough both rotor + drums up (both sides of rotor if you can get to it) as long as they aren't like corduroy like Paul says.

 

~Jason

[Stevenc22]

You guys inspired me to go check the rear hardline for collapse and to blow out the rear hard line. Did that but it didnt fix the problem.Kinda interesting the fluid when i blew out the rear line came out blue/black even though i have been pumping gallons of clear fluid through the system when i bleed.

 

[Stevenc22]

Another update. I started disconnecting the rear hardline working from the back to the front. At  7 psi the t joint would not bleed and so i then disconnected the rear hardline from the master and at 7 psi the master doesnt even leak fluid. How would you ever gravity bleed if the master doesnt even dribble at 7 psi. I wonder if the master piston is stuck at the back of the master.

[TobyB]

Quote

This isn't a pad issue. I have multiple pads and switch them out and didnt change anything. It's a clamping force issue.

 

Have you put hydraulic gauges on your bleeders?  Strain gauges on your caliper bridges?

 

Because Fout has to equal Fin * leverage, and the booster, IF IT'S WORKING, should give you about 800 psi

at the calipers, which, with 1975 brake pads, was about 200 psi short of lockup on a set of Michelin XAS's.

After that, your leg can get it up to about 2000psi just sitting in the shop (sweating), or probably quite a bit

more when you see the ass end of that semi coming at you...

 

Modern pads grab better, but modern tires do too, so fortunately, the 800 psi is still a pretty good place for the booster to stop helping.

 

But if you're not getting fluid out of the rear circuit, you're possibly also not getting fluid to half of your fronts, so, well, that might be your problem right there....

...because that will HALVE your leverage on the front pads, and 400 psi is, yeah, as you say, about 70% of a good stout 'Phew!' stop.

 

This isn't a 76 by any chance, is it?

 

t

 

[conkitchen]

30 minutes ago, TobyB said:

 

This isn't a 76 by any chance, is it?

 

Thinking the proportioning valve fitting?