Drum brakes all around. Rears not releasing. I read a plunger in proportional valve may be sealing off fronts or rears. No light is on but it may not work. I am looking for the correct part number to source a new one before I start to disassemble the original proportional valve. Any thoughts?
Firstly I want to say thank you to the VCVC studs that have kept this site up and running for years. It's been years since I logged on... Thank you again. So I read that there is a piston that will move and direct fluid to the brakes that build pressure as a safety against a brake line blowout. Latter models had a reset button to recenter the piston and deactivate the brake light. I checked for continuity at the fault switch at the valve and there is continuity. I'm not sure if that would trigger the light to come on if the circuit is normally open... but my brake light is not on... any way, I was thinking that when I swapped out the rear end with another 2nd gen and had the brakes bleed, the piston in the proportional valve may have shifted locking up the rear brakes all the time and also back pressure is causing excessive drag at the rear drums. I was going to try and remove one of the end feeder lines coming from the master cylinder and see if I can move the piston until it clicks into the nub ground on the shaft to reset the fault switch and allow fluid to distribute more evenly to the front drum brakes and release the back pressure to the rears drum brakes... Has anyone else experienced this issue with the drum brakes? (see attached photos)
Its not exactly the same, but you get the idea...
I may try this instead
There is no "recenter" button. The only way to get the Pressure differential valve - - proportioning valves ar only on disc/drum or disc/disc brake set ups.
the only way to get a PDV to recenter is to properly bleed the brakes - - - all of them, beginning with the passenger rear, driver's rear, passenger front, driver's front. If the valave does not re-center, it should be replaced.
Although I have converted to front and rear disc brakes, I still have the PDV (52 years old)
You may need to bleed the brakes a couple of times to get the PDV to re-center
I suggest if the valve will not re-center, to remove the valve, and clean it internally with brake cleaner
Last edited by joyrde; 03-21-2019 at 06:43 AM.
Start by cleaning out the wheel cylinders. Every time I've had this type of issue on our vans, it's been brake mud in the wheel cylinders. 50 years worth of crud builds up and the pistons inside the wheel cylinder will apply but not release. Here's a shot of a 1st gen master cylinder that has that kind of issue. This is from Frank's van (65Dream).
Frank, this is from June of 2015, can you believe it!?!
If you see any evidence of this kind of brake mud, it's time to clean out or replace all the brake system hydraulics. I never throw out the old wheel cylinder hardware unless it is beyond repair. I've found that the newer brake parts are poor quality. I clean up and inspect all the wheel cylinder parts. Our exact size is hard to find. BTW, I've yet to find a bad USA made rubber cup. When I clean them up, they are still pliable and reusable. I use them on my vans. The wheel cylinders are another story. They can get pitted beyond repair. The springs inside can rust beyond repair but the pistons less so. The safe thing to do is get replacement rear wheel cylinders if you don't want to mess with it. I think the 68 Camaro will fit but the pistons are smaller in diameter.
108VanGuy...
1969 Chevy Panel, 250 CID, 3 Spd.with OD, 3.36 "WedgieVan" Daily Driver
1967 Chevy Panel, 230 CID, 3 Spd. 3.36 "UtiliVan" 292 TFI coming. Owned since 76
1964 GMC Panel, 194 CID, 3 Spd. "CrunchoVan"
1965 GMC HandiBus Custom, 194 CID, 3 Spd. "MilkVan" Seized Engine
1965 Chevy Panel 350 CID, 3 Spd. "RustoRoof" Runs but wiring bad
1969 Chevy 108 Display 307 CID THM 350 Power Brakes 3.73 Posi
1965 Chevy Panel, V8, 3 Spd. "Gold Hills Van" Best body of my 65s
1965 CamperVan, V8, 3 Spd.
I did a disassembly of the valve, got the piston to move again, thoroughly cleaned, reassembled and re-bleed the entire system and it appears to be working well and also a more balanced stop off all 4 wheels. Wheel cylinders and master cylinder were replace about 8 year and 2 years ago respectively. Thanks for taking a look... Also unlike newer models, it appears if you have a brake hose blows out and you hit the brakes, the greater pressure on the good line will move that piston to the center striking a metal tip that looks like a straight screw, causing a ground and the light to come on on the dash, maybe? I am not totally clear on this design.. What I noticed is that spring pressure moves the piston back to the home position and does not seal off the lines that are damaged so that any further braking will loose fluid... Is that your understanding? Newer models seem to shut off the flow to the blown lines. I attached some photos where I used a drift pin to move the piston towards the center from both the front brake feed and the rear brake feed. They both were under strong spring pressure. I should have taken a picture of the safety sensor to, sorry about that...
When I did get the pistons freed up, I failed to notice if it is just one solid piston on that shaft that can move to the left or right. It felt as if they move separately, but that would meat they move independently along that center shaft which now that I think about it, that would need an inner seal along that shaft so I believe it is a solid shaft with a piston at each end and a spring at the end of each side. I did not want to try and press out the brass tapered flair at the large end to completely dissect as I not sure if that was possible. Anyway hope that helps someone down the road.
So you said you still have a PDV (52 years old), but Not in use on your van? I changed all my flexible brake hoses and cannot get a good pedal even after installing another Master Cylinder. I do not how leaks any where yet I keep accumulating air. I suspect something wrong at the PDV but I did get fluid through the fronts when I let the front brake hoses hang into plastic containers with some fluid in the containers. Is your PDV for sale if not in use? or is there a source for them? I am running out of ideas...Originally Posted by joyrde
No need to respond, just got a mechanic neighbor to help with the bleeding and I'm good to go, Thanks
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