is the factory brake proportioning valve tunable?

[Frankenchevy]

this is a kind of obscure boring technical question. I'll attach all the angles of the proportioning valve. it is the one mounted to the back of the front crossmember below the front of the engine. it has two incoming lines on the top from the brake master cylinder and a sender here:

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from what I understand, it allows pressure to build in the rear drums before applying pressure to the front, but I may be wrong. it has a line out to each front caliper and a single line to the rear brakes.

fronts

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rear

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on the rear there is a piece threaded into the large hole, then the brake line threads into that. is there anyway to tune this valve to work optimally for rear disc conversions? either through the piece that threads into the large 3/4" port that eventually leads to the rear or is there something adjustable here:

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that "nut" was covered by a rubber dust cap. I'm not sure if maybe the valving sits behind that or if it's even modifiable?

thanks!

 

[77 K20]

http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.pdf


I have spent months researching 14 bolt FF rear brake conversions. And I'm still annoyed with the whole thing.

Found dozens of people who just "slapped" discs on the rear and everything worked great.

In theory it seems you should either modify the stock combo valve or go buy one that is setup for disc/disc. Then in theory since the rear disc brake cylinders displace more fluid then your stock master cylinder doesn't have a big enough reservoir for the rear brakes. Some say then the proper thing to do is get one where the reservoir is split 50/50 in volume for front to rear. Then some still add an adjustable proportional valve to the rear brakes.

I'm still fiddling around with mine as I don't like much about it.

 

[Frankenchevy]

mine work fine, but I want to make it right. which calipers did you try?

 

[77 K20]

I matched the calipers... so my rear are identical to the front ones.

 

[77 K20]

Depending on what brackets you bought for the rear calipers you might need to keep an eye on the inner pad wear. My brackets like to jam up the caliper causing it not to free float. So all the braking is done only with the inner pad. It typically lasts only 7,000 miles.

 

[Frankenchevy]

Depending on what brackets you bought for the rear calipers you might need to keep an eye on the inner pad wear. My brackets like to jam up the caliper causing it not to free float. So all the braking is done only with the inner pad. It typically lasts only 7,000 miles.

thanks, I'll look at that. I was looking at this at summit:

https://www.summitracing.com/parts/sum-760185/applications/year/1986/make/chevrolet/model/k30/

I want to buy a new master cylinder any way just because of this ones age. did you find what they say will work?

maybe mine works well because its hydro boost and not vacuum boost. the hydro boost works really good and the master cylinder is different for hydro setups. again, I also have the secondary proportioning valve in the rear, but I'm not sure if any of the factors are actually substantial.

 

[77 K20]

The brake proportioning valve from summit should work- but you might just try modifying your stock one first. It would save you $50 + shipping.

I wish I had hydroboost brakes. I've thought about switching to them but have other things to work out first.

I went with some old corvette with front/rear disc brakes. That was supposed to be the popular one. The brake pedal travels farther than I like as it has a smaller diameter piston in it.

 

[Frankenchevy]

The brake proportioning valve from summit should work- but you might just try modifying your stock one first. It would save you $50 + shipping.

I wish I had hydroboost brakes. I've thought about switching to them but have other things to work out first.

I went with some old corvette with front/rear disc brakes. That was supposed to be the popular one. The brake pedal travels farther than I like as it has a smaller diameter piston in it.

Which year vette calipers? Did you swap master cylinders too?

 

[MtBraun]

I used the Summit PV with my rear disc brake conversion. I took me a while to get everything bled but they work great now.

 

[Frankenchevy]

I used the Summit PV with my rear disc brake conversion. I took me a while to get everything bled but they work great now.

The one in the link above, correct?

 

[yevgenievich]

Check if there is a spring for the rear circuit inside the prop valve. If design is similar to the one in my firebird, I have increased the amount of rear pressure by changing to a stiffer spring

 

[rpcraft]

Just a suggestion but this is a 4 wheel disc proportioning valve...

https://www.carolinaclassictrucks.com/74-96-Workhorse-Van-prop-valve-4.html

 

[MtBraun]

The one in the link above, correct?

Looks like the one, yes.

 

[Frankenchevy]

Check if there is a spring for the rear circuit inside the prop valve. If design is similar to the one in my firebird, I have increased the amount of rear pressure by changing to a stiffer spring

where did you source the spring?

 

[Frankenchevy]

Just a suggestion but this is a 4 wheel disc proportioning valve...

https://www.carolinaclassictrucks.com/74-96-Workhorse-Van-prop-valve-4.html

that looks identical to the summit one I linked earlier. I wonder if there are any differences besides the price...