rear brakes/ prop valve

[Mlevair]

I recently removed the proportioning valve guts and put more pressure to the rear and now I can lock up the rear brakes. The hole car squats under braking now. I tried limiting the front pressure with a prop valve but that just killed the front brakes. The original prop valve just does not give enough pressure to the rears.Just thought I would pass along a cheap/free fix to get good rear brakes.

[p8ntman442]

Did you check stopping distances to get any idea wether your Butt Dyno is correct?

[Mlevair]

I didnot check 0-60 stopping times as I did this to my autocross car. I did loose about a second off of my times in an autocross which is alot, especially when your at the top of your game. I then did it to my street car but I forgot to do before and after times. I just wanted my car to stop! I started wearing out rear pads which I have never done before.

[The Dark Side of Will]

earlier this year there was a big discussion of exactly how the prop valve functions. Did you find and read that?

[Mlevair]

No I did not look at that thread Iam new to this list I did research and development on my own. I race cars more than I talk about them.

[The Dark Side of Will]

phpBB/viewtopic.php?t=3757

MUCH discussion of the Fiero combo valve.
Probably not relevant to you if you just completely gutted yours.

[p8ntman442]

I did not check 0-60 stopping times as I did this to my autocross car. I did loose about a second off of my times in an autocross which is alot, especially when your at the top of your game. I then did it to my street car but I forgot to do before and after times. I just wanted my car to stop! I started wearing out rear pads which I have never done before.

Sounds like some decent results. I assume you have stock rear brakes right? Ever tried using the grand am rotors/calipers on the rear? I switched to the GA setup in the back, and can lock up the back end with the stock proportioning valve intact.

[Mlevair]

I ran grand am brakes before I took out the prop valve but it didnt seem to help. I took them back off because they were heavier. I run 14 inch wide slicks on the back so its a little different than stock.

[Pyrthian]

did you remove the entire guts, or just the spring?
I have the grand am brakes up front, and stock rears - so I got WAY to much front. sofar, I just pulled the spring from the prop valve. maybe I should pulled the plunger too....

[JohnnyK]

Are you SURE It won't lock the rears first in a high speed panick stop? I'm going through the exact same thing with the mustang right now, as I'm installing rear discs.. What I did was steal an sn95 master cylinder/dist. block, but the original plan was, if you're not going to run a dist valve/proportioning combo, run the adjustable prop valve to the rear brakes. Of course this might not relate to GM, but one can only assume it does.

[p8ntman442]

Here is an excellent write up of a guy installing a Wilwood adjustable proportioning valve in an F-body.

EDIT: link http://www.thirdgen.org/propvalve

Now since the fiero has the prop valve already there, and has 4 lines running off it, Im sure you would have to do some creative bending and flaring to put one of these in, but I cant imagine it being that hard. Then just install a T block for the fronts, and or A second adjustable valve in the front circuit.

For about $120 bucks you could have GOOD adjustment of your brakes and test them under track* conditions.




*track conditions means you dont experiment with brakes on a populated road, seeing as none of you are fluid engineers.

[The Dark Side of Will]

Link?

I don't know what you're looking at, but every Fiero combo valve I've ever seen has two inputs and three outputs. One of those outputs feeds both rear brakes. Plumbing in a rear prop valve shouldn't be hard from a plumbing standpoint... just a packaging standpoint.

[p8ntman442]

2 inputs on the fiero, front and rear right?

So run the rear out line from the master to the prop valve, and run the front out to a T that feeds both fronts, problem solved, unless you blow a line, then your sol.

[The Dark Side of Will]

Oh, you're talking about the master cylinder. I was talking about the combo valve. The combo valve has the shuttle valve and the prop valve in it. If you just gut the factory prop valve and install an inline prop valve downstream of the combo valve, you'll have fully adjustable rear bias, full shuttle valve functionality AND wouldn't have to redo nearly as much plumbing as you would if you'd gotten rid of the combo valve completely.

[Mlevair]

On my race car I put in an adjustable prop valve in the rear and I always had it wide open so in my street car I didnt put one it and in a panic stop I can lock up the rear brakes but it doesnt get squirly.
I just bought a car with grand am rears and cross drilled fronts and stock prop valve and my stock car with no prop valve stops better. I may do some 0-60 tests this week and see the difference.