Chevy Express 3500 Hydroboost with sinking pedal

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Just started today, strangely right after battery was replaced, chalking it up to coincidence. Anyway the pedal feels fine at first and the brakes work great but as you approach a stop the pedal keeps sinking lower and lower till it feels like it's going to hit the floor. Brake warning light does not illuminate. The master cyl and everything around it are wet, not dripping just sort of a sweat. The firewall inside is dry. The wheels are all dry. The brake fluid is a tad low. Hoping I can just replace the brake master cylinder and won't have to mess with the hydroboost, I do have factory service manuals and will read up on it tomorrow but in the meantime any tips appreciated. Happy New Year

Miller Challenger 172Miller Thunderbolt AC/DC 225/150Miller Maxstar 150 STLVictor 100CVictor JourneymanOxweld OAHarris O/ASmith O/A little torchNo, that's not my car.
Reply:Looks like my biggest issue will be bleeding the ABS module without a Solus scan tool. I have AutoEnginuity but only with the Ford bundle, it can only do generic stuff on other vehicles.Miller Challenger 172Miller Thunderbolt AC/DC 225/150Miller Maxstar 150 STLVictor 100CVictor JourneymanOxweld OAHarris O/ASmith O/A little torchNo, that's not my car.
Reply:Sounds like a bad master cylinder, if so you can bench bleed it, we had a 97 3/4 suburban that was horrible as far as brakes, big block with hydro. I replaced everything in the brake system trying to get it to stop properly, if they got wet in the rain you could forget about it stopping.IMPEACH BIDEN!NRA LIFE MEMBERUNITWELD 175 AMP 3 IN1 DCMIDSTATES 300 AMP AC MACHINEGOD HELP AMERICA!“Freedom is never more than one generation away from extinction.We didn’t pass it to our children in the bloodstream".RONALD REAGAN
Reply:i have a 2001 silverado 3500 dually duramax.  last year went thru same as you.  i think if when bleeding brakes you almost need the scanner.  replaced brakes pads, master cylinder, you might get buy if you don't bleed brake pedal to floor.  its when air goes thru abs or proportioning valve when you need scanner.lincoln 125spmonkey wards 250 ac/dcmiller 211 w/spool gunahp  200 sx tiglotos ltp5000dof course duramax diesel
Reply:i forgot to add, flush out and change the hydro boost power steering fluid.  that also can make a differencelincoln 125spmonkey wards 250 ac/dcmiller 211 w/spool gunahp  200 sx tiglotos ltp5000dof course duramax diesel
Reply:Well I decided to take it to a shop right by the house. They got the master cyl installed but when they went to do the bleeding they found a pinhole in one of the brake lines that runs from the ABS module to one of the rear wheels, it had been rubbing against the transmission cooling line and wore through. First they tried to fix it with a kit but after 3 hours they gave up on that, it's in a very tight spot. The local Chevy dealer doesn't have one, now everyone is closed. That truck needed to be on the job Thursday morning, now I'm going to have to go there early Thursday morning and unload everything we need to do a 400 amp service. They are skeptical about even finding that line anywhere due to the backlog because of the strike. Looks like we might be working out of a pickup for a while.Miller Challenger 172Miller Thunderbolt AC/DC 225/150Miller Maxstar 150 STLVictor 100CVictor JourneymanOxweld OAHarris O/ASmith O/A little torchNo, that's not my car.