I have a 91' Z28 with 2.73 posi with drums. Later on this week I will be swapping the out rear for 00' LS1 rear w/3.42.

First of all, I would like this to be as hassle-free as possible along with the least amount of down time.

After searching for awhile its seems that since I have a 91' it should be okay to use the stock proportioning valve since they were almost identical to all-disks brake setups. Could someone correct me if I am wrong?

I also read that I could use a adjustable proportioning valve from summit, jegs or wilwood. However, I don't want to flare anything or buy any different lines and try to adapt fittings.

So basically, I would like to know is there anyway that I can keep the stock stuff and still have my brakes function properly?

Also is there any certain way I should bleed the brakes to make sure they stop right?

Thanks in advance,
Jay

 

Anyone?

 

The stock prop valve is actually called a 'combo valve'. For street driving safety, you want to retain that combo valve because it has a few functions for safety that just a regular prop valve does not have- the safety dummy light letting you know you have proper fuild pressure, and a metered rear bias valve to allow a delay in reaction of the front brakes which allows the rear pads to come into contact first to prevent any high speed yawing of the car before the fronts come on. this helps the car track straight under hard quick brake pedal inputs.


Taking this off and replacing it with a prop valve will eliminate the saftey light and the residual valve effect of the stock combo valve. Not advised for daily safety. With changing fuel loads, tires applications and pressure changes, etc, a standard prop valve not always adjusted optimum could potentially land you in trouble in a panic stop.


I would advise keeping the factory combo valve for street safety. If a more desired performance level is wanted (more agressive rear bias) then go to a post 1990 1/2 1LE combo valve (J65 valve). They canged the thread pitch mid year in 1990 so make sure youget a later one for your 1991 car.

 

I found a way to install an adj. prop valve with minimal cutting and flaring. Just need two pieces or pre flared ISO bubble, a pipe bender and flaring tool. I also was able to keep the idiot light, and be able to go back to stock if I wanted to. I kind of like it this way because I don't believe any stock valve will lock the rears on dry pavement.

https://www.thirdgen.org/forums/brak...-cylinder.html Scroll to post #9.

Just FYI I have my valve pretty much backed all the way out to keep the rears from locking up before the front. Have a friend watch while you skid to a stop from 40mph and adjust it so the rears NEVER lock before the fronts.

 

Thanks for the help guys,

I think I rather just go with 1LE setup. Eventhough the way Trump has it setup doesn't look to bad.

What all would I need to convert to the 1LE setup? Anything else besides 1LE combo valve?

Thanks,
Jay

 

I don't know much about the LS1 swap, but I think parking cables are the only big gotchya. I'm sure Ebmiller88 would be able to hook you up with whatever you need.

 

I went with a prop valve out of a 91-92 1LE car. Retains everthing stock on the car, lines and hookups. I here you can use the stock drum one, but if your gonna do something, do it right.

 

both cars use the same combo valve. I did the 98 rear swap and it works very well with the factory combo valve!!!!!!!

 

I dunno if they are the same, the part #s were different for a stock RS prop valve (rear drum) and the 1LE prop valve (rear disks).

 

maybe the v8 rs's, I compared mine to a disk car and were the same, 91-92 were supose to be the only once that used the same combo valves for both rears, eaither way it will work well with out changing it out

 

My wheels will all easily lock if I lay on them hard- and thats with a factory 1LE combo valve.


I read from your other post you disabled the guts on the standard combo valve- That was my post years ago under afrikingoodtime. With the internals removed from the rear and the plug installed, the fronts begin not to work properly due to the way the fluid pushed through anfd the balance springs are not counterbalnced properly with the rear spring setup.


What rear brakes are you running and why do you have to turn them doen so much compared to the C4 fronts. If ou are running C4 fronts and stock rears, you should not have to turn down the weaker rear setup unless your valve is not metering right and the fornts are loosing pressure. Read further dowen that post I made years ago and you will see where I later suggested people not to do that plug mod because the fronts loose pressure. I will bet you can not lock your fronts right now since you are having to bandaid the rears so much by dialling everything out of them. I would suggest either complately removing the factory gutted valve,OR taking the aftermarket prop valve off and going to just the 1LE valve- you'll see much better results.

 

Thanks thats just the info I was looking for.

Jay

 

My brakes work wonderfully, probably the only thing on the car that doesn't give me fits and I can lock the fronts up if I want to. I'm running the stock iron rear calipers.

I don't believe the rears need all that much pressure, the adjustable valves can only reduce the pressure by 57%, thats why I have backed out so far. Also the rear end of these cars are light so I don't think you would need a lot of braking in the back to get them to lock.

The only thing, as far as I know, the factory valve does is reduce pressure, act as a distribution block, and connect an idiot light. Metering is only needed in a rear drum setup.

 

I underdstand all that- I will resay what I said before in hopefully a way you understand better.

The stock iron calipers front and rear will lock all four wheels with the 1LE combo valve- I had this setup on my car, I know it works.


NOW- You have still the same rear iron caliper disc setup AND you supposedly installed better C4 front brakes-BUT- you now have to turn down the stock iron rear calipers to balance the C4 fronts where you did not have to with the stock fronts and no valve mod. So this says your bigger C4 front brakes are WEAKER in proportion to your stock rears? I don't think so, your frotns are not getting adequate pressure because of the way you left your combo valve dismantled internally. I will guess you only dismantled the rears plug on the combo valve and not the fronts section also. There are metering springs on both sides that counterbalance eachother for bias.


You should not have to turn down the small weak iron caliper stock rears when you have supposedly more heavy duty larger C4 fronts- You should be turning down the fronts- You have a front pressure problem.

I will guess you are exerting more than normal pedal pressure to get your setup to lock. If you push anything hard enough, it will eventually lock.


I gaurantee I could lock all 4 of mine on the freeway at 100mph with the pressure you are exerting to lock yours at 5mph, This is not a slam, this is an education to you- I wrote that 'prop valve mod' post years ago and tested it- I know the downsides first hand.

 

I guess I didn't think about how my system relates to others, but you're right, the C4 setup has LESS piston area so it uses a higher system pressure for stopping which I can feel in the stiffer pedal. Its not really a problem its just how it works, I actually like the pedal feel. I guess this gives the rears more pressure than before with stock fronts, which is why I need to turn it down more than others.

There is no front bias spring, the fronts get full pressure from the master, it justs splits to each wheel. I took a part the whole combo valve when I did this to clean and learn how it worked. The lines for the rear travel from the MC to to the proportioning valve and spring. The idiot light is actuated from a movable piece between the front and rear ports. Any significant pressure difference grounds the signal.

 

Ya know, you can tell people till you're blue in the face and some people are just too ignorant to heed a warning for their own good.


There ARE metering springs in the front of GM combo valves. You are wrong. I tried to help, you learn the hard way when your front brakes finally fail.


http://www.gnttype.org/techarea/brakes/combo.JPG

 

I don't know why you're trying to start an argument here. As I said my brakes work great, especially the fronts. How could I have a front pressure problem if they lock?

Metering is used on drum brake equipped cars. My prop valve has NO spring, metering, nothing between the M/C and output ports. I took it apart so I know exactly whats in there.

http://auto.howstuffworks.com/master-brake2.htm

 

Yoiu'ere not getting it. I am trying to warn you of a safety concern. Your brakes may work now, but they are fading and will keep reducing front pressure. Disc brake combo vlaves meter also. The GM combo vlaves for our cars all have metering on the fronts.

I too have tinkered with these combovalves- I mentioned to you earlier that was my post you got the initial info from. I also know through testing what happens to these combo valves in due time when you disasemble them like you and I both did.

For your knowledge, I am a suspension expert and happen to hold a NASCAR licence actively as a crewchief- I do know what I am talking about. Suit yourself. Its the last I'll post if you think I am offending you.
I wish you luck.

 

What about people running adjustable prop valves like in the tech article section?

https://www.thirdgen.org/propvalve

This is more or less what I've done. I don't understand what metering will do for me, but add a little lag time from the fronts kick in.