Brake issue

[82chevdiesel]

Im having an issue with my brakes, when i first drive the truck after its been off for a little while (could be hours, days, doesn't matter) and Ii apply the brakes it seems as if the fronts are engaging before the rears. Im not sure if this is whats actually happening but it jerks the truck forward with only a slight amount of pedal pressure. After the brakes have been applied a few times they operate as normal. The brake setup is hydroboost with rear drums front disc. I was think maybe prop valve or air in the lines? What do you guys think?

[Sabaka454]

Residual check valve in the master or sticking metering valve in the combination valve can cause what you are decribing. Do you have a quick take-up Master?

[82chevdiesel]

I dont think I have quick take up, im running the stock calipers so i wouldn't think the p.o. would have installed one.

[hatzie]

Download the 1982 service manual PDF from the thread in my sig. Look at the brake troubleshooting section starting on PDF page 525.

"Grabby brakes. Faulty spool action caused by contamination in system. Flush steering system while pumping brake pedal."
--Hydroboost troubleshooting page 525 1982 GMC Light Truck service manual

[chengny]

Check that the shoes in the rear drums are correctly installed. A full axle brake set comes with 4 shoes that look almost identical but 2 of them are the leading (front/secondary) shoes and 2 are the lagging (rear/primary) shoes.

Because the steel frames are identical and only the size of the lining difffers, it is easily possible to install them incorrectly. Any number of wrong installations can cause severe brake shudder especially on the first couple of stops or when it is wet/humid out. You can have the leading and lagging reversed on one or both wheels, 2 leading shoes on one side and 2 lagging on the other, etc.