Bad brakes kinda day

[Stompah]

So driving home from work today in 80+ degree weather I come off the highway and drive through downtown to drop a friend off. As I am going up a long semi steep hill the tranny is getting a little warm but the engine temp remains below the half way mark.

Halfway up the hill is a stop light. I cannot explain the feeling of what happened. The pedal didn't quite sink to the floor nor was it spongy. There was a little resistance till about half way down. I recognized what was going on and did a quick double pump then the brakes caught hard.

I continued up hill till I was able to pull into a closed library. I drove around and there brakes worked normally.

I drove it home slowly and leaving enough room that if they failed and the parking brake didn't stop the truck I'd be able to put it in the ditch.
I get home and park it. I pump the brakes and it will sink right to the floor with moderate effort. Reservoir is full and I see no external leaks. The master cylinder looks non original to the truck but I am unsure of its age.

It is a 1998 Suburban 2500 with the 454 and hydroboost. This is my first hydroboost. I have noticed the past few times I drove it that of I steer with the brakes pressed I can feel feedback through the pedal.

Any ideas where to start looking?

It's bad timing for me as I don't have alot of time to dedicate to chasing a mechanical issue...

 

[Dr.Zoom]

I'm not familiar with the hydroboost so maybe someone else will come in with a better answer, but have you checked the power steering fluid?

 

[SAATR]

Master cylinder bypassing internally. Best way to check is to get a couple of the proper size plugs and block the master off and test it. It should get hard as rock And have no give whatsoever. But they are pretty inexpensive, and if it's an unknown it's a good place to start.

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[Justin S]

If the truck is off and you press the brake and hold it, does it sink to the floor?

 

[Stompah]

I'll check PS fluid level and the engine off brake check here in a bit. Gonna be some time before I can plug the mastercylinder.

If I go the plug test route might just buy a new MC since it would be easy to swap out and I'd likely have to bleed anyways.

 

[Justin S]

If the truck is off, and the pedal sinks to the floor, and you have no external leaks, i'd say master cylinder.

 

[Stompah]

PS reservoir is full. Engine off brake pedal goes down 4/5s of the way then stops.

 

[Ruger_556]

Master cylinder bypassing internally. Best way to check is to get a couple of the proper size plugs and block the master off and test it. It should get hard as rock And have no give whatsoever. But they are pretty inexpensive, and if it's an unknown it's a good place to start.

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What he said ^^^