Brake troubleshooting

[Jayred]

I have a braking issue where I have to stand on the pedal to get the thing to stop and when I do stand on the pedal if I'm not careful the rear drums lock up. I was going to replace the booster but I'm not sure if maybe I just have the wrong proportioning valve or possibly both? I have front disc and rear drums but I'm starting to wonder if I have the wrong proportioning valve. I was thinking about installing a classic industries PB1108 black anodized adjustable valve, but I don't know if I should mess around with these adjustable ones. Just trying to save some money and not buy a whole new booster and valve assembly. Thoughts?

Truck is a 71 C10 with front discs and rear drums.

[pjmoreland]

What master cylinder and what proportioning valve do you have? Post a photo if you can.

[HO455]

Have the brakes always been like this or has something changed?
Have you tried bleeding the front brakes?
Have you tested the booster to see if it has a leak?

[Jayred]

I don’t remember when the problem started, truck has been garaged for about 5 years and I’m bringing it back to life. I just remember the brakes were super sketchy. The proportioning valve was new but I’m not sure if the problem started after I swapped it. I’m just wondering if I put the wrong one in. When I get it running I’ll try the booster check. I’ve never diagnosed a bad booster. I remember the pedal being like a rock at slow speed where I had to stand on it. At faster speeds if I pressed to hard the rear would lock up, fun as hell but dangerous. I was going to buy a whole new master/booster/valve from classic industries but i would rather not blow all that money if I don’t have to. I’ll try to post a pic. Thank you

[pjmoreland]

Everything you've got there looks pretty normal to me. Are your front brakes working? Maybe you could do a basic functionality test by jacking up the front end and have someone push the brake pedal while you turn the front wheels by hand. Maybe your brakes need to be bled.

[HO455]

I would look at the proportioning valve after all other components have been verified as good. Proportioning valves are pretty bulletproof especially compared to other components in the system.
After 5 years of no use any part with rubber pieces is suspect. And hopefuly I'm not to being too negative, but there may be more than one problem with the system.
You could have a master cylinder problem affecting the front brakes and a bad hose to the rear axle causing the rears to lock. (Been there last summer)
If the hard pedal problem isn't easily remedied you ma need to get a brake pressure guage to find the problem.
Good luck and keep us posted.

[Jayred]

It didn’t even occur to me that the front brakes might not be working at all. I’ll start with the least expensive fix (new brake lines) and go from there. Thanks for the replies, this literally saved me days of troubleshooting and a few bucks too. Stay safe ya’ll

[Tx Firefighter]

I've seen front brake hoses swell shut internally and cause the same issue. Rock hard pedal and rear locking up. New hoses fixed it.