I need a definitive answer on this before i put my car on the road tomorrow.

I swapped a 85 Disc Rear into my 80 Drum Rear car. I have an 80 Master Cylinder, and 80 Proportioning Valve.

I've been told that i can use the 80 Proportioning Valve if i take off the Nut on the back of the prop valve, take out the red widget thing and spring in there, and put it back together. And that this will allow me to have proper brake pressure to the rear brakes.

is this true? If not, what on earth should i do.

sorry to post yet another thread on this, but i'd rather not have any doubts about my braking system (I rather enjoy living )

-Kurt

 

It makes sense to do that. All proportioning valves limit brake pressure to the rears to avoid rear brake lockup. (Yank the e-brake, I think we all know what happens... The back end comes around and becomes the front end.)

However a drum rear/disc front type valve also has a component in it (I forget the name... I lost my brake reference book as well. As soon as I find it or remember the name I'll tell ya.) that holds a little bit of pressure to the rears to equal the return spring pressure... This way as soon as you step on the pedal, the drum shoes begin to move rather than waiting for pressure to build up and overcome the springs...

When you switch to discs, you don't need that little bit of pressure since there's no return spring. In fact it'd be bad because effectively, you'd be dragging the rear brakes a little all the time. Hence the removal.

Personally, I'd get a new proportioning valve for a disc brake car, but that's just me. I don't like to monkey around with brakes and do patchwork.

 

Drum brakes use a pressure residual valve to keep 10psi of fluid pressure to the rears when you take your foot off the pedal. It is located I though on the bottom of the brake master cylinder.

The proportioning valves has to do with braking bias of fluid pressure when you apply the brakes.


Maybe that is what you guys are talking about, but they are 2 different items and it seemed to me that you guys made them out to 1 and the same.



John

 

hell, i put a 84 lsd into my 80. didn't change or do a thing to the proportioning valve. have no problems at all. works great. the only thing that sucked was converting the line to fit the 80.

 

There is a difference.Drum brakes require far less pressure to work because of the larger friction material area and the leverage provided by the shoes.Thats why drums make better parking brakes,they can work well with just cable pressure,whereas discs require thousands of pounds of fluid pressure to clamp down hard on the disc.
A drum prop. valve will work with disc brakes,but you wont be getting full braking action to the rear discs.You may or may not notice it,but it is there.
Pele and rhinor61 are right about the residual pressure valve.It will make the rear discs drag.10lbs may not be much,but it will make the pads hold enough to increase wear/heat.

 

It isn't worth monkeying around with brakes... Just upgrade the valve and cylinder to 85 type...

 

so is the residual pressure valve in the master, or the block with all the lines going to/from it right next to the master (the prop valve)?

and if it actually is in the master, i'll go remove it, but then what the hell did i remove from the proportioning valve (a little red widget, and a spring, under a nut that i used a 7/8" socket to remove, becuase i didnt have my metric that large)... the nut is facing the firewall, on what i assume is properly called the proportioning valve.

 

You're calling it right, but I believe the pressure hold up dealie is built into the proportioning valve.

I'd either hit the scrapper for a master cylinder and valve or use your existing ones as cores on rebuilts from the store... As you said... You like living.

 

*sigh*

well aside from the big nut on the back of the prop valve, from which i removed what i assume to be the thing that shoots the rear brakes the lower pressure... there is also a nut on the bottom of the master, where the rear brake line comes out before it goes to the prop valve.

the rear brake line threads into this nut... i guess i'll just bite the bullet and remove the master to see whats under that nut.

i dont have the luxury of getting an 85 master/prop valve because i've already spend a buttload of money trying to make my brake lines work with what i have.... so i'm gonna have to figure this one out and use it for now (or forever if i can get a concrete answer on what exactly these 2 widgets do.

damn the uncertianty.

 

I am having the exact same uncertainty with my car. I removed the spring from the bottom of the master and the spring from the rear section of the prop valve. Unfortunately the pressure to the back brakes is so great that the entire car won't move. I didn't remove the red thing from the prop valve. Will that work? How did you make out with this?