Sorry- another rear disc brake question

[77 K20]

I have been thinking about doing this for about a year. Recently had the brakes heat up pretty bad going down a very steep trail, so pulled the trigger and just ordered the DIY4X rear disc brake brackets and then new brake lines from ORD.

Now I still don't understand what I need to do with the brake proportion valve. Some say it might be ok. Some say no, need an adjustable one. Some say change the master cylinder instead to one that was made for disc/disc brakes. Some then say you need a master cylinder for disc/disc but it has to be a LARGE bore because you need extra fluid volume. If a newer style master cylinder is used, then my stock proportion valve can stay installed?

I feel stupid for asking this- but after hours of searching it seems like every truck is different, so everyone has their own way of "fixing" it. I don't see why every truck would be different.

 

[MadOgre]

I would think an adjustable proportioning valve would be the least complicated way to go. probably $ wise too.

 

[MadOgre]

If you can keep it simple, why not?

 

[marks86]

just put the brakes on and drive it, if your back brakes lock up like mine did, installe a adjustable prop. valve afterwards

 

[firebane]

I have been thinking about doing this for about a year. Recently had the brakes heat up pretty bad going down a very steep trail, so pulled the trigger and just ordered the DIY4X rear disc brake brackets and then new brake lines from ORD.

Now I still don't understand what I need to do with the brake proportion valve. Some say it might be ok. Some say no, need an adjustable one. Some say change the master cylinder instead to one that was made for disc/disc brakes. Some then say you need a master cylinder for disc/disc but it has to be a LARGE bore because you need extra fluid volume. If a newer style master cylinder is used, then my stock proportion valve can stay installed?

I feel stupid for asking this- but after hours of searching it seems like every truck is different, so everyone has their own way of "fixing" it. I don't see why every truck would be different.

Honestly with how brakes work you shouldn't be using much of your rear brakes at all going down a hill since if your prop valve is working correctly you should be getting 60% or so braking on the front rather than the rear.

I would be looking into better pads, rotors and brake fluid and ensuring the brake system itself was in tip top shape before dropping a lot of coin on a rear disc setup.

 

[friscobob]

This discussion reminds me why Physics should be a mandatory subject in high school. Can we say hydraulics children?
Bob

 

[marks86]

its a know fact that when adding rear disc to a chevy truck it will create more pressure in the rear and into makes the rear brakes lock up. regardless of what the stock prop. valve should be doing or what physics say. I know from first hand experience that when you add disc brakes to the rear of a 73-87 you should use a rear adjustable prop. valve
end of story

 

[MadOgre]

its a know fact that when adding rear disc to a chevy truck it will create more pressure in the rear and into makes the rear brakes lock up. regardless of what the stock prop. valve should be doing or what physics say. I know from first hand experience that when you add disc brakes to the rear of a 73-87 you should use a rear adjustable prop. valve
end of story

Yep its because the disc set up is more efficient at producing stopping power thus the need to adjust the proportioning of pressure. THAT IS PHYSICS!

 

[77 K20]

I have taken physics and a hydraulics class thanks.

And from that I know the pistons on a disc brake are larger than a cylinder on a drum brake. There will need to be more volume of brake fluid needed to move the larger piston. And then as the brake pad wears out and the large piston is getting extended even more fluid will be needed. This is why GM made reservoirs for disc/disc brakes.

I also have read that some master cylinders have residual pressure valves built in to them. Drum brakes are 10 psi and disc brakes are 2 psi. Does the stock master cylinder I have now have this?

So that being said.... what will fit in the truck without re-plumbing every hard line in the truck.

And to keep it simple then wouldn't it be best to buy a proportioning block that is fixed like this one?

http://www.summitracing.com/parts/rsd-pvk72/overview/year/1977/make/chevrolet/model/k20


And as far as a master cylinder what years/vehicles did GM have 4 wheel disc brakes, but NOT use hydroboost? And would that master cylinder then be able to bolt on to my truck without modification?

 

[77 K20]

Honestly with how brakes work you shouldn't be using much of your rear brakes at all going down a hill since if your prop valve is working correctly you should be getting 60% or so braking on the front rather than the rear.

I would be looking into better pads, rotors and brake fluid and ensuring the brake system itself was in tip top shape before dropping a lot of coin on a rear disc setup.

My system in the rear is not in tip top shape. I've always had issues with the right rear drum snagging/locking up/dragging. Haven't been able to fix it, and when it comes down to it it isn't worth fixing. Since I off road a lot drum brakes fill up with sand/dirt/water/mud and then are useless. Disc brakes clean much easier. Also once a drum brake heats up it takes forever for it to cool.

 

[77 K20]

just put the brakes on and drive it, if your back brakes lock up like mine did, installe a adjustable prop. valve afterwards

I don't suppose you remember a part number of your valve, or where you bought it from?

And why did you order an adjustable one vs just a fixed one?

Did you have any issues mounting it?

One concern of mine is since I have the ORD steering box brace it bolts into the crossmember where the prop valve sits. Currently it fits just fine over the stock one, but with an adjustable one it might be an issue getting up there and adjusting it.

I was hoping to learn from peoples past experience in what worked for them.

 

[firebane]

I don't suppose you remember a part number of your valve, or where you bought it from?

And why did you order an adjustable one vs just a fixed one?

Did you have any issues mounting it?

One concern of mine is since I have the ORD steering box brace it bolts into the crossmember where the prop valve sits. Currently it fits just fine over the stock one, but with an adjustable one it might be an issue getting up there and adjusting it.

I was hoping to learn from peoples past experience in what worked for them.

I noticed this very thing myself while working on my truck and thing its a very stupid design. I honestly would be moving the prop valve up to the master cylinder and running a couple of new lines.

 

[marks86]

I don't suppose you remember a part number of your valve, or where you bought it from?

And why did you order an adjustable one vs just a fixed one?

Did you have any issues mounting it?

One concern of mine is since I have the ORD steering box brace it bolts into the crossmember where the prop valve sits. Currently it fits just fine over the stock one, but with an adjustable one it might be an issue getting up there and adjusting it.

I was hoping to learn from peoples past experience in what worked for them.

my father had one sitting in one of his drawers, its a ford racing one I know shame on me, it was a absolutely piece of cake, I did it after work in the middle of winter

I too had the ORD steering brace which completely covers the stock prop valve, I mounted it next to one of my batterys, one end went to rear brake line, other end went to stock prop valve simple.

if you read through my build thread you can spot it(its white and says ford racing on it)
http://www.gmsquarebody.com/forum/showthread.php?t=9868

 

[77 K20]

Looked up more stuff, cross referenced, and made my best guess. Ordered the master cylinder and prop valve. The master cylinder has large equal reservoirs which would be a good thing for identical disc brake pistons front and rear. Also on the description is says for rear disc brakes.... and it was not a hydroboost unit.

ACDelco 18M1887 Professional Durastop Brake Master Cylinder Assembly

Right Stuff Detailing PV72 Disc Brake Proportioning Valve

 

[marks86]

Looked up more stuff, cross referenced, and made my best guess. Ordered the master cylinder and prop valve. The master cylinder has large equal reservoirs which would be a good thing for identical disc brake pistons front and rear. Also on the description is says for rear disc brakes.... and it was not a hydroboost unit.

ACDelco 18M1887 Professional Durastop Brake Master Cylinder Assembly

Right Stuff Detailing PV72 Disc Brake Proportioning Valve

can you post a link to the stuff? I wanna check it out