Rear brakes seem to engage first

[1425956]

I am having a little trouble with my rear brakes being the first brakes to engage. My pedal is very touchy and when I depress it to a certain point, it grabs the rear brakes all of the sudden. If I back up and hit the brakes, the fronts will lock up. If I am moving forward and slam brakes, the rears lock up.

My brake fluid reservoir is full of fluid. Any ideas?

[7T2]

Is it disc/drum, or drum all the way around? Have you done anything to the brake system recently?

Sounds like something wrong with the proportioning valve/residual valve.

[ont_guy_38]

Leaking wheel cylinder/axle seal...any oil on the brakes will make them grab as soon as the pedal is touched

[tucsonjwt]

ditto ont guy 38. I had leaking "wheel seals" aka "axle seals" twice on my 3/4 ton floating rear axle due to defective design OEM GM wheel seals. I got the newer 2 piece design installed and all is well so far. If you have a 1/2 ton, then most likely leaking wheel cylinders are the problem. Once your drums are soaked then you likely will have to get new drums (you might luck out if the leak is just from wheel cylinders.)

[1425956]

Sorry for the lack of info. It is an all stock 84 GMC Sierra Classic 1/2 ton with disc/drum brakes. The truck sat for 9 years, and everything else I have had problems with has been seal/gasket related due to drying out.

Should I just replace the proportioning valve? Or should I pull the rear drums off and inspect for any leakage first and clean? I am pretty familiar with this body style of truck, however, I have never really messed with drum brakes on any vehicle.

The brake system has not been touched in at least 9 years. And I see no fluid or signs of a leak.