Proportioning valve issues. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mpierce9 · 01-13-2015, 09:05 PM

I ran into a similar issue a few weeks back. My solution was
1. Clamp down the Combo valve button.
2. Remove the MasterCylinder cap
3. open the rear bleeders (both sides)
4. monitor the MC fluid level
It took 48 hours to get good fluid out.

All this should be able to be resolved with a two person bleeding job much quicker. I was told that slow 1 pedal pump bleeding works best because the volume of air stuck in the line after making such major changes.

I was making the mistake of doing a two person bleed job allowing the person that pumped the pedal to pump it up multiple times to build pressure. This was causing my issue that filled the line with air and didn't allow fluid to the rear.

**MikeB** · 01-13-2015, 09:58 PM

The latter half of this video is a good demonstration of how the metering valve works. All it does is delay pressure to the front brakes. But you can see how both front ports should have the same pressure.

I still don't have a real handle on the inner workings of the prop valve, but I know that it limits maximum pressure to rear drum brakes in a disc/drum vehicle. Of course, you're stuck with the preset pressure determined by the spring.

https://www.youtube.com/watch?v=PlElcEGjfd8

83swb · 01-13-2015, 10:12 PM

Found a possible issue tonight. Somehow (dumbass me) didn't realize that I had the lines switch from the master cylinder to the valve, had the front reservoir feeding the back and back feeding the front brakes. BUT the bigger issue was the way the valve itself was machined. The valve is machined for a 3/16" line for the rear inlet (should be 1/4) and the front half is machined for a 1/4" (should be 3/16").
So this valve is getting sent back to summit and I have a line on a factory valve to put in its place.
So beware if buying a CPP combination valve for your truck, may have been mishap in the machining process.

**MikeB** · 01-14-2015, 07:55 AM

That's seems odd. Can you post pictures of this valve and its part number?

01-13-2015, 09:05 PM	#1
mpierce9 Registered User Join Date: Nov 2013 Location: ATLANTA Posts: 223	Re: Proportioning valve issues. I ran into a similar issue a few weeks back. My solution was 1. Clamp down the Combo valve button. 2. Remove the MasterCylinder cap 3. open the rear bleeders (both sides) 4. monitor the MC fluid level It took 48 hours to get good fluid out. All this should be able to be resolved with a two person bleeding job much quicker. I was told that slow 1 pedal pump bleeding works best because the volume of air stuck in the line after making such major changes. I was making the mistake of doing a two person bleed job allowing the person that pumped the pedal to pump it up multiple times to build pressure. This was causing my issue that filled the line with air and didn't allow fluid to the rear.

01-13-2015, 09:58 PM	#2
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,670	Re: Proportioning valve issues. The latter half of this video is a good demonstration of how the metering valve works. All it does is delay pressure to the front brakes. But you can see how both front ports should have the same pressure. I still don't have a real handle on the inner workings of the prop valve, but I know that it limits maximum pressure to rear drum brakes in a disc/drum vehicle. Of course, you're stuck with the preset pressure determined by the spring. https://www.youtube.com/watch?v=PlElcEGjfd8 __________________ Mike 1969 C10 LWB -- owned for 36 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- converted from 250-six to roller cam 350, Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!

01-13-2015, 10:12 PM	#3
83swb Registered User Join Date: Sep 2012 Location: CT Posts: 133	Re: Proportioning valve issues. Found a possible issue tonight. Somehow (dumbass me) didn't realize that I had the lines switch from the master cylinder to the valve, had the front reservoir feeding the back and back feeding the front brakes. BUT the bigger issue was the way the valve itself was machined. The valve is machined for a 3/16" line for the rear inlet (should be 1/4) and the front half is machined for a 1/4" (should be 3/16"). So this valve is getting sent back to summit and I have a line on a factory valve to put in its place. So beware if buying a CPP combination valve for your truck, may have been mishap in the machining process.

01-14-2015, 07:55 AM	#4
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,670	Re: Proportioning valve issues. That's seems odd. Can you post pictures of this valve and its part number? __________________ Mike 1969 C10 LWB -- owned for 36 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, recent AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- converted from 250-six to roller cam 350, Vortec heads -- sold 1981 C10 Silverado LWB, 305, TH350C -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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