I've been having the same problem. I bled the brakes, replaced the master cylinder, flushed the system with new fluid, and bled the ABS. I'm considering the brake booster. I've never had one go out before. Ideas??

 

I know this is an old thread... But for those that have bled and replaced everything did you ever figure out your problem? I'm having the same issue.

 

having the exact same issue, pedal sinks after starting the truck (1994 K1500) been reading like crazy and checking YouTube but getting nowhere,, did you fix your truck and if so what was the problem?? Basically had 2 leaking rear wheel cylinders, changed them out , bled like Crazy for 2 days , tried bypassing rear abs, getting nowhere, pedal feels good until I start the truck,, it will stop but takes a few pumps to feel right

 

I changed the brake booster and master cylinder about to bleed the cramp out of it and see what happens front calibers were changed a year ago

 

I bought the Innova 5610 Diagnostin Scan Tool, it attaches via the OBDII Port under dash. You can use the ABS bleed or Caliper Test if you have changed calipers. it takes a few times but it is very easy but yes a helper is still needed. my tester was bout on Ebay for $199.99

 

This is a generic response as several people have posted in this thread with the question of how to get all the air bled from the system on earlier model trucks.

On pre-2000 GM trucks (and even into the early 2000’s for C3500 models) with a rectangular master cylinder, the combination valve utilized a plunger valve to meter brake fluid to the rear brakes. The lines route from the MC directly into the combination valve. This valve is present on 2 and 4 wheel drive models with and without ABS - they just look slightly different depending on the year and options.

The plunger valve must remain depressed during the entire bleeding process to allow complete flow through the valve, otherwise it’s nearly impossible to bleed all the air out of the system.

The plunger can be found under a black cap that’s typically facing the front of the truck but may also be facing other directions depending on the orientation of the valve. See pic below.




There is a special tool for keeping the plunger depressed, however a large hose clamp or zip tie and a dime will also work.

Some combination valves have 2 plungers on them, however only one of them will move. You’ll need a helper to press the brake pedal while you watch/feel for movement from the plungers to determine which one needs to be clamped.

 

I know this is 4+yrs old but my 98 having the same issue. And one guy told me his booster was causing the spongyness. he put a 02 booster on and it works great and if abs is deactivated but not deleted it can cause this as well. I've also bled my brake till the legs were numb and no go. So those two ideas above are my next as my abs light is on and the abs sensor is cut and taped