Car is an 87 drum, swapped in a 00 rear with disc. Bled brakes, car stops like garbage. Fronts get super hot and rears are so so... I bought a wilwood prop valve but it doesn't make sense to me to install it in the rear line, which is what I've been reading for the last hour...

Help please!

I have an '86 IROC. After swapping out the original rear diff that was drum and 2.73 ratio for a 2002 Camaro SS Auburn unit with 3.23 ratio. Ratios not too important here, but I do pay attention to detail. I had no issues with braking with the original 1986 MC and prop valve, except slight nose dive. Nothing major though...

If your brake fluid is 20+ years old, your MC is probably headed south, not to mention your prop valve. Open it up. Brake fluid absorbs moisture. Once water is in your brake system sh*t starts rusting internally. HTH

I did suck out the fluid, but not enough to need to bench bleed it and replaced it with fresh. Stopped better before with drums.

I thought drums required more fluid so the rears should be locking up, shouldn't they?

First, drum brake system activated by wheel cylinders take much less brake fluid when the friction linings start to wear, compared to disc brakes. Especially multi-piston calipers. Consider the surface area of the caliper pistons vs wheel cylinder pistons. Wheel cylinder pistons are much smaller and take less fluid...
Anyway, either you have air in your brake system. Or your MC is going bad, which is common on any hi-mileage vehicle, specially after any changes to the brake system that repositions the MC piston within the MC. Fluid could be internally bypassing the internal seals.

So you don't think this is a proportioning issue?

You may have multiple issues. Too many variables to diag over the forums. Sorry.
I can't and you can't look inside these individual parts. You have some parts to consider in fixing your issue in your rear end swap:
1) The master cylinder may have failed or may be failing
2) The proportioning valve has failed
3) some other problem, i.e. improper brake pads, slide locked/frozen etc.

After several stops from 60 mph, front rotors are 160ｰ and rears are about ~120ｰ.

Seems to Be each wheel end is doing its job. I think more grabbing power from the rears IMO

Proportioning valve goes to the front line (the rear brakes), and the original prop valve needs to be bypassed; hope that helps...

Now we are getting somewhere..

But my question remains, why does it go on the rear line when the rear line is the one that needs more fluid?

Are you getting confused maybe on a brake system like that of a top fuel dragster?
That is straight line, with a 'chute at the end of a run.
Our cars have to corner. Also, consider if the road is wet or iced. If there is too much brake in the rear vs front, say 40%F-60%R, the *** end is going to come around on you. If you had 0%F-100%R, you're going in a straight line when hitting the brakes.
Is this right?

I've been thinking...

Maybe the best thing to do is to get rid of the combination valve that includes a metering valve, which we no longer need since it delays braking to the front to compensate for the drum shoe movement.
Then plumb the front brakes lines together with a simple T from the MC out to the front calipers. Then put in your adjustable prop valve from the rear line out of the MC going to the rear, which then T's out to both calipers on the rear axle.

Yes... So I didn't know of the delay in the valve. This would also explain why before the swap when cold the brakes would lock up but delayed Manor...

So the MC PROVIDES equal pressure to both front and rear once metering valve is gone.

Thanks, that's what I was curious to as why are we cutting back pressure to rear..

Ty

I think we almost have it.
I think the reasoning behind the engineers preferring at least 60%F-40%R braking, is because during moderate to heavy braking (with the factory suspension) most of the weight is being transferred to the front and the down-force on the rear wheels is lessened. So in this case, too much brake in the rear is not a good thing.
A lowered vehicle with adjustable shocks & struts might help achieve the 50%F-50%R you & me are looking for.

I've been doing alot of research into lowering my car 1" front and rear. To do it properly, we need lowering springs, adjustable struts and shocks, an adjustable torque arm to adjust pinion angle, adjustable pan-hard bar to center rear axle, relocatable rear control arm brackets to adjust instant center of gravity so no wheel hop.
After pricing the parts, its at $1200+. This is a project I'm getting into next summer after I pay my off my current mods.

GL, let us know how it goes!

Buy a 1le prop valve. Direct bolt in and cheaper and easier then the wilwood once you factor in the cost of all the correct fittings and ends. Plus have to cut an reflare brake lines can be annoying.

even the 1LE prop valve is a compromise: GM will not release a production car to the public that will have the rear end lock up under weird braking conditions with Joe Average Commuter behind the wheel... that's a whole slew of lawsuits waiting to happen, so they bias the braking heavily towards the front. i'd bet that even the factory backed teams that ran 1LE cars in road racing wither used a different stiffer spring or an adjustable prop valve.

had the exact same problem when i swapped the disc brake rearend out of an 02 into my drum brake equipped 86: the rears would be dead cold after a hard drive while the fronts were really hot.

i fixed it by taking the spring out of the prop valve- which then put too much fluid to the rear brakes- and putting an adjustable prop valve in the rear brake line just after the stock prop valve, which allowed me to turn down the pressure to the rears just a bit...

you could also find a much stiffer spring to put in the stock prop valve, but the adjustable valve gives you the ability to fine tune it to your particular car and driving style.

when you take the spring out of the stock prop valve, make sure you keep the aluminum cup that it seats on inside the nut in there, and jam it up into the nut where the spring used to hold it or you will have a leak.