Hi Scotty I have a 1974 gmc Jimmy that just hit 100000 miles it has disk brakes in the front and drum brakes in the rear.
I started getting a soft pedal so we bled the brakes we replaced the brake booster and then the master cylinder and now when trying to bleed the rear brakes we get no fluid so we replaced the brake proportioning valve we still have no rear brakes but the front disk bleed just fine please help me get my baby back to life
PS LOVE YOUR CHANNEL AND ALL YOUR WORK IVE BEEN A FAN FOR YEARS AND HAVE WATCHED TONS OF YOUR VIDEOS
Might be time for new wheel cylinders, bleeder ports may be corroded shut. Also check brake hose, they can come apart inside and clog up.
The bleeder ports are ok I pulled them completely out and blew through them (sorry should have added that) my next stop was to go down the brake lines to each connection and disconnect them to see if I could get get fluid from anywhere. It would be odd for both the rear wheel cylinders to give out at the exact same time would be my luck tho. I will have to look into that thank you very much for the help!
On a vehicle that old everything in the brake system needs to be treated as suspect unless known to be recent. You've obviously got a clog somewhere in the rear hydraulic circuit, so work your way back and find the last place that fluid comes out. If that turns out to be at the proportioning valve I'd replace the entire hard line with NiCopp and put in a new rubber brake hose. Wheel cylinders are cheap, I'd change those out as well unless they're very recent.
if the fluid was in there too long it can make the brake lines corrode inside too.
I think you may find the issue at the proportioning valve.
It sounds like its pressure differential piston is getting pushed to the rear and closing off the rear brake outlet.
Easy enough to test the theory. Loosen the Rear Brake Inlet line going into it and step on the brake pedal. Brake fluid should squirt out so you know the valve is getting brake fluid from the master cylinder's rear brakes reservoir.
Tighten that back up and do the same to the Rear Brake Outlet line on the valve. See if there's brake fluid squirting out of there when you step on the brake pedal.
If there isn't, you have to recenter the pressure differential piston inside of the proportioning valve body.
There's plenty of stuff on the internet on how to do that.
Once you get it centered, you may want to buy a proportioning valve bleeder tool.
You remove the warning switch and install the tool while bleeding the brakes.
The warning switch has a spring loaded pintle that sits in the groove of the differential piston. When the piston moves the pintle gets pushed up (and the brake warning light turns on).
The tool is rigid. It sits in the groove and doesn't allow the piston to move off center.
When you're done bleeding the brakes you remove the tool and screw the brake warning switch back in.
