G-Body Disc Disc Proportioning Valve Question

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7883Elko

n00b
Aug 7, 2018
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1
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Hi there so I recently purchased a QP 9" with disc brakes for my 83 Elko and have it all setup now except am having problems bleeding the brakes I should note that I had to change the brake line that runs from front to rear T during rearend swap. I have some pedal but its really not much compared to what I had with my drum brakes setup. I realize now at the minimum i need to get a new proportioning valve for disc-disc setup, but is there anything else I should cover while I'm at it? I've also read that you can simply modify the stock disc-drum prop. valve for disc-disc just by removing the small rubber piston from the isolation valve. Can anyone actually confirm/deny this? The instructions for it are here:

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

Not trying to be cheap, prop. valves aren't crazy expensive, but if its true I wouldn't mind doing that just to get it back on the road asap! I know there's a bunch of posts about converting to disc disc setups but I haven't seen much about modifying current prop. valves for it, but if there is could someone please drop a link for it!

Thanks!
IMG_4096 (2).JPG
 
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spidereyes455

G-Body Guru
Mar 6, 2013
776
1,614
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Northeastern PA
When I converted my GP to 4 wheel disc I used a prop valve from an 83 or 84 el Dorado , since they are a similar size and weight to a Gbody, and it worked perfectly. A Toronado or Riviera of the same vintage with 4 wheel disc would also be the same.
 
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81cutlass

Comic Book Super Hero
Feb 16, 2009
4,639
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You can't convert a G body prop valve disk/drum to disk/disk.

There are 2 styles of prop. valves, a kelsey hayes and a bendix (I think). A g body has a bendix and a kelsey hayes can be modified. I might be backwards on what a g body has and what can be modified, but either way a G body has the style that can't be modified.

Honestly, I wouldn't even bother getting a disk/disk prop valve. I did and it leaked from day one.

Ended up finding Summit sells an adjustable prop valve that is the same price as the fixed disk/disk style. It looks exactly like the wilwood thats twice the price so it's a safe bet summit buys them from wilwood or more likely they buy them from the same mfg.

https://www.summitracing.com/parts/sum-g3906/overview/
 
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7883Elko

n00b
Aug 7, 2018
3
1
3
Thanks for the info guys. How did you mount the adjustable prop valve? That is more of the way I am leaning towards except I was sort of looking for a quick fix for the time being so I don't have to redo my brake lines until winter probably.
 

84GP455

G-Body Guru
Jun 19, 2007
779
95
28
Methuen Ma.
I changed my rear end a few years ago it's an 87 GN and it had the disc swapped on already. After it was installed I wasn't getting enough pressure to the rear brakes, so I bought a 50/50 prop valve for around $100 and put it in and the pressure was good all around after that. I think our stock proportion valves are a 70/30 combo, meaning 70% goes to the front and 30% goes to the rear so you need to change to a 50/50 prop valve because the discs require more pressure, I believe. I can get you a part number if you need it later, but I'm at work right now otherwise I would have posted it for you.
 
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84GP455

G-Body Guru
Jun 19, 2007
779
95
28
Methuen Ma.
I had an old adjustable prop valve hanging around so I tried that first but didn't like the fact that you have to cut your brake line in order to install it. Then it really didn't adjust anything, I couldn't get the pressure to go to the rear with it at all. I wouldn't recommend one.
 
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Bonnewagon

Rocket Powered Basset Hound
Supporting Member
Sep 18, 2009
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Not only would I get a correct 4 wheel disc valve, I would get the correct 4 wheel disc master cylinder too. I had 4 wheel disc on my '77 TA and I had both items installed. The result was incredible braking with no worries at speed. You can use the ugly cast iron 2nd gen master or an aluminum 3rd gen- the prop valve is pretty much the same for both. Since I see a blower sticking out of that Elky I would advise going first class on the brakes.
 
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malibudave

Greasemonkey
Mar 12, 2010
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Houston, TX
What calipers are you running in the rear? They are most likely the a stock front, metric caliper. If it is a single piston caliper, with no emergency brake provisions, it is most likely a 2.5"/2.38" piston, metric, D154, front caliper. If it has eBrake provisions, it is most likely a early 80s Eldo caliper with that has a 1.875" (1-7/8") piston.

The main issue is not the prop valve, but the volume of fluid needed for the rear calipers. Stock drum wheel cylinder have a 3/4" bore and a .44 square inches of piston area. The rear calipers are most likely the the same calipers as your front calipers. A front caliper with a 2.5" piston will have a piston area of 4.9 square inches. That's a big difference in volume needed to fill and build pressure. The reason there is a longer pedal stoke is because the smaller master cylinder has to move more volume to the rear calipers to fill them up and then for them to build pressure. For a vacuum boosted setup, I would go with larger bore master cylinder. 1.0" bore would be the minimum, but with a boosted setup, you may need to go with a 1.125" (1-1/8") bore. For a manual brake setup, I suggest a 24mm bore master cylinder with a large reservoir with 4 wheel disc brakes and 4 metric, D154, front calipers.

Most universal disc/disc prop valve should bolt in place of the stock prop valve. The inlets and outlets of the universal prop valve should be the same as the stock prop valve and should be in the same location.
 
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fleming442

Captain Tenneal
Dec 26, 2013
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What calipers are you running in the rear? They are most likely the a stock front, metric caliper. If it is a single piston caliper, with no emergency brake provisions, it is most likely a 2.5"/2.38" piston, metric, D154, front caliper. If it has eBrake provisions, it is most likely a early 80s Eldo caliper with that has a 1.875" (1-7/8") piston.

The main issue is not the prop valve, but the volume of fluid needed for the rear calipers. Stock drum wheel cylinder have a 3/4" bore and a .44 square inches of piston area. The rear calipers are most likely the the same calipers as your front calipers. A front caliper with a 2.5" piston will have a piston area of 4.9 square inches. That's a big difference in volume needed to fill and build pressure. The reason there is a longer pedal stoke is because the smaller master cylinder has to move more volume to the rear calipers to fill them up and then for them to build pressure. For a vacuum boosted setup, I would go with larger bore master cylinder. 1.0" bore would be the minimum, but with a boosted setup, you may need to go with a 1.125" (1-1/8") bore. For a manual brake setup, I suggest a 24mm bore master cylinder with a large reservoir with 4 wheel disc brakes and 4 metric, D154, front calipers.

Most universal disc/disc prop valve should bolt in place of the stock prop valve. The inlets and outlets of the universal prop valve should be the same as the stock prop valve and should be in the same location.
I would also like to add that if you got QP's base disc kit, it is indeed the Seville/Eldo calipers. They are a mf'er to bleed. To aid this: cycle the e-brake a few times in between bleed sessions. The pistons ratchet out as you use the e-brake, thus reducing piston travel to the rotor.
 
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