luke4907

iTrader: (5)

OK before you guys ream me a new one for doing this, I'll ream myself one. I'm sure you've heard about this mod if you're reading this page. I'm talking about

1) opening up the plug in the front of the proportioning valve
2) pulling out the piston and spring
3) spitting on it and cursing it
4) plugging the hole in the "plug"
5) reinstalling the plug.

I realize this mod is stupid for several reasons; I'll list the biggest ones I can think of:

1) too much braking pressure to the rear wheels will cause a spin out, and possibly severe injury or death
2) If the mod gives you extra pressure to the rear brakes, you may exceed the specifications of the rear braking system and/or hydraulics and cause them to fail
3) The mod may simply cause them to function worse than they used to.

Please keep in mind I am a moron for trying to resolve my braking issues in the order I have, and by no means do I recommend anyone else perform this modification.

What I should have done before touching the proportioning valve (and what you should do before being stupid like me) is to check the entire rear braking system as follows, but not limited to:

1) Check the hydraulic lines (even though a visual won't be conclusive) from the prop valve to the calipers for blockages or kinks.
2) Check the calipers and caliper guides. Make sure you can move the caliper freely along those guides, and make sure the caliper's piston can be pulled out freely by hand, as well as compressed back into the caliper using a C-clamp
3) Check the friction materials. Check the rotors and pads to make sure they are evenly worn from side to side and between each pad, and to make sure the friction material is within factory specs (compound and measurements).
4) Check the master cylinder for leaks and contaminated brake fluid. Replace and fix as necessary.
5) Check for proper brake booster operation. With the engine off, pump the brake pedal until it feels firm, and hold. Then start the engine. If the pedal starts to travel down, the brake booster is working properly.
6) Check the struts, springs, and other suspension parts. If these are worn or damaged, they will not be able to manage weight shifting between acceleration and braking, drastically increasing the distance required to stop your car.

Hopefully I've covered all safety bases here.

luke4907

iTrader: (5)

Now I haven't done 1-6 above, however I plan on still checking all of the above out to see if I find any problems with the rest of my braking system. I just chose to do this first because it seemed to be the least amount of work with the greatest chance of improving the functionality of my rear brakes (even if that means too much rear brakes).

The reason why I knew I had a problem with my brakes is because I jacked my rear wheels in the air, started the engine and kept it idling, put the tranny in gear, and stomped as hard as I could on the brake pedal. My rear wheels would not stop turning. I decided personally I would risk having too much rear brakes and have to switch things back to stock over than not trying to fix this dangerous problem I was experiencing at the time.

Here's what I did: I drained as much fluid out of the master cylinder as I could. Then prepared the engine bay for brake fluid spills (as in rags everywhere on driver's side). Then took out the plug from the front of the prop valve, followed by the spring and piston. found a washer to fit between the piston and spring, which further compresses the spring about 1/4". I put the piston, washer, and spring back into place and put the plug back on. Started the car, had a buddy press the brake pedal and bled the air out using the plug. Finally I tightened the plug and checked for leaks for about 5 minutes. All was well.

Here's the results: The instant I sat down in the seat and put my brake pedal down, I heard a creaking noise (heard over the engine and exhaust) coming from the rear of my car I had never heard before. It happened each time I pressed the pedal. Now this creaking will probably tell you something about the condition of my rear brake parts, so keep that in mind. Yes I know I'm a moron, please go to the top of the thread and begin reading again if you missed the part where I admitted that.

After carefully testing my brakes in my driveway to make sure they still worked, I took my car for a test drive. I could feel a difference in the car's stopping power. My 2 friends observed as I slammed on my brake pedal, only the fronts were locking each time. Only when I pulled my parking brake and slammed on the pedal would the rears lock (and I've never been able to get the rears to lock in any situation, wet or dry), yet there was no fishtailing on dry pavement. I am personally very satisfied with this result, however, I accept that the conditions of my rear brakes or the tension in the spring may wear down and I will have more rear brake problems in the future. I just want to post my results for everyone to review because I would have done this awhile ago if I had known about it. Feel free to post replies with comments on whether modifying the prop valve has worked for you or caused you additional problems, and hopefully this will help some of you keep your car in safe operating condition.

iansane

iTrader: (1)

Why not just get rid of the prop valve and run 100% pressure to the front and get a $30 adjustable prop valve plumbed into the rear. It'd be a good time to plumb a linelock into the front.

luke4907

iTrader: (5)

That's a great idea once I know the rest of my system in up to snuff. I also need to get some brake flaring tools and get some practice in making new brake lines before I start trying to do this on my car. It's a daily driver so I can't really have any downtime with it.

Jetmeck

iTrader: (3)

You are already running 100% pressure to the fronts through the factory proportion/combination valve. If you remove the combo valve the brake light will remain on or not ever come on ( I forget which) if something goes wrong with your brakes. Granted most of us would know anyway but I prefer to have the light operational. You can gut the rear side of the valve and add the adjustable valve after the factory one and keep the light.

iansane

iTrader: (1)

HAve you pulled apart the combo valve? Once you gut one side, the other side isn't functioning. Is uses those springs you pull out to tell when pressure is lost between the two.

Stock pressure isn't 100% to the front or there wouldn't be anything on the front side.

If you can't tell when you lose brake pressure, run out and buy a MC for a 4thgen car 98 and newer.

MrDude_1

stock it does put 100% of the pressure straight thru to the brakes.. it only goes thru the valve so that it can compare for the light.


that is a great idea.
the 98+fbodies have the prop valve switch in the brake master cyl.. so you can replace the crappy prop valve with a good aftermarket one, and the light still functions.

iansane

iTrader: (1)

Really? Learn something new everyday. I thought the whole plunger assembly pulled it down a little percentage.

Jetmeck

iTrader: (3)

I like this idea, I assume it will bolt up ok ? But what about the size of the MC in relation to the brakes it was made for and the ones on our cars. This deserved further review.

Like I said in my first post "most of us would know anyway" ( if you lost brake pressure. But some of us want the light to be functional anyway. A preference type of thing.

MrDude_1

the displacement of the two master cyls is identical.



however, it isnt super critical anyway, but i can understand peoples paranoia about changing brake parts.... lol

Jetmeck

iTrader: (3)

How about the brake light switch, it is on the master itself ? It senses fluid loss out of the master itself. A couple of butt splices
should do just fine ?

Jetmeck

iTrader: (3)

If this works out this could be a sweet setup. A line lock in the front line and an adjustable proportioning valve in the rear all the while keeping the factory idiot line functional and completely doing away with a known problem on these cars , no more combo valve.

MonkeyWrench

the valve has two diff parts to it a bias safety valve that if the pres diff between the front and back system are to far apart it closes the lower presser one so if you blow a brake line on the back the valve switches over turns the brake light on and cuts it off so you still got front brakes then the second part behind that screw that you take out for this mod is a proposining valve to reduce the brake press to the rear brakes and if you remove the stuff behind there you send the same press to the front and back and the bias valve will still work but you got to be careful if you are braking hard in a corner cuz you will lock up the back and be in I world of trouble

don't know if that ansers any q's hope it douse

Dirtbik3r

Proportioning valve decreases stopping power by restricting pressure to rear brakes from 0%-50%. Basically, If there is 3000psi at the master cylinder, 3000psi gets applied to the front lines and at 50%, backs gets about 1500psi letting the fronts lock up first. Not to be confused with a metering valve. This system usually replaces the combination valve which contains 3 parts.
1. Metering valve (reduces nose dive)
2. Brake switch
3. Proportioning valve

Summed up, a proportioning valve restricts flow to the back brakes while reducing stopping distance due to more friction on the road by the rear tires.

iansane

iTrader: (1)

I'm pretty sure most everyone in this thread knew that... Could be wrong though?

Randy91

I did the "Shim the spring" mod. Now I am tring the "Plug the hole" mod. I'll post back in a day or two on the results. I may bleed the whole system, before I make a final determination. The 2000 "LS1" rear is in the garage as a brake parts donor. Brakes are going on a 3.42 rear to go in my 1991 Formula. Buts that's a while off yet. I do have a spare junk yard PV that I got to see what's in them. From what I can tell shimming the sring will not help, although I thought it did, it might even hurt braking. MC connects to the spring side of the valve pushing it closed along with the spring? I'm not sure what GM intented to do there.

Jetmeck

iTrader: (3)

I have a shim under my spring also and I think it helped a little, get back to us on your results.