Here's the background:
I have a 1990. All at once I installed a Wilwood prop valve, 949 stainless lines, late caliper brackets, calipers and unknown late-style (street) pads and rotors.

I have a Motive-type bleeder, and I used Motul RBF fluid. I've bled and bled the brakes, I've adjusted the rear adjusters. Even with the prop valve set to full open, I cannot get the rear brakes to lock first. I assumed it was the junk street pads/ rotors I was using.

I just swapped on a set of Centric discs and Porterfield R4 pads... same result. The car brakes well, but no rear lock-up even set to full open. I can see that the rears are working, just not enough to lock first.

Can calipers become 'less effective'? I do have a second set of calipers I can swap on, but after bleeding a fully empty system, I'd like to see if this is even the right direction...

I *should* be able to get the rears to lock up, right? As far as I know, this is how you set the prop valve...

 

Mine behave the same way until you get the car on the track and get some heat in the pad.

 

That may be just the answer I'm looking for.

I have my first track day with this car on Monday.

 

Odd. My old xp8 front and rs4 rear would still adjust to where the rears could lock up first. With the weaksauce hps pads up front, the rears had to be backed off a decent amount.

 

Full open is max brakes to the front. Full closed (clockwise) is full rear. I will explain how a bias valve works later when I am not on my phone.

 

With my 11" rotors in front and 10" in back, and HP+ all around, with the prop valve allowing the max rear biasing, I couldn't lock the rears worth ****. In fact, at an autocross event the brakes were awful and locked up way too quickly.

Eventually I got tired of the brake squeal of the HP+s on the street, and threw in $20 duralast pads int the front. the pads were horrible and had no bite or modulation, but i could back off the prop valve and get a pretty damn good balance to them for once

 

I don't have a 'true' bias set-up, it's simply the adjustable proportioning valve that replaces the factory (fixed) prop valve.

This is my first set of race pads on the street. They are noisy, but it's not the end of the world - but, man, are they dusty.
Running more aggressive pads on the rear may be an answer - I'll run it as it is for this track day and see how it goes. I do want to sort it out correctly eventually, though.

 

+1, although I have race pads in the back, so they're no good when cold, and better than the front HP+ when hot. I was locking all four at my track day yesterday, and this is still with 1.8 rotors in the back. An upgrade to sport rears should be soon.

 

If you are running the valve full "open", aka full counterclockwise with lots of thread showing, you are not running any rear brake at all, and the brakes are going to suck.

I've got the big 11.75s in front w/ DTC70s, 10" 1.8 rears w/ DTC60s, and I can add enough rear bias to make the car uncomfortably twitchy under brakes.

 

We were missing each other on this.

I meant 'full open' as the valve (internally) was fully open and not restricting flow in any way. That's full in.

Sorry for the blury pic, I just ran out to make sure I didn't have it backwards.

 

I did about 175 miles yesterday on the Lightning course at NJMP.

The brakes worked well for me all day. I had a few quick front lock-ups under hard braking, but not the rear. I left the **** screwed in all day long.

Maybe it's the pad compound, maybe there's still a bubble or 2 in the rear lines/ calipers. The rear wheels are just as dust covered as the fronts, so I can see that they're working.

From karting, I know that when I've had too much rear brake, you'd end up going backwards off the track. This is the first time I've had this car on the track, so I don't have much of a baseline to go by.

 

Don't relate it to karting, that's a solid axle and an entirely different story. Plus it's generally rear braking only on the smaller karts.

 

not do disagree with some one who has 4 million posts but, if you have to much rear brake in any car you will end up going *** backwards off the track. doesn't matter what kind of rear end you have.

 

any car will do that. you can still see F1 cars start to swing their rear end when they lock up the rear brakes.

Not sure why you aren't getting the locking up, but I would hope its not because you have an air bubble in the lines. Anyways, nothing to complain too much about if you're still stopping fine on the track.

 

definitely, I've done my fair share of ebrake turns. Solid axles certainly don't help though. The point was that it's difficult to accurately compare karting and full sized car driving.

 

Grabbed this from another thread:
https://www.miataturbo.net/showthrea...873#post774873

This will be my next step. I used the motive bleeder 'top-down', and I didn't hit anything with a hammer.
Since my system was completely open (new flex lines, calipers, prop valve), I'll bet my issue must be hidden bubbles that were missed during all my top-down bleeding.

 

I have the same issue on my car. I'm running a '90 with the MSM sport brake package and the wilwood prop valve, some wagner thermo quiet pads up front and some EBC Reds on the rear.. The fronts still lock up first with the valve fully open aka same pressure in all 4 corners. I still haven't taken it to the track to get some heat in the pads like a few of these guys suggested so that's going to be my next step before changing other things on my setup. I started a thread here or on CR.net you might be able to reference.