So tell me about hydro boost, 1988 K30, pedal just about going to the floor

[57NAPCO]

Brake light is on and the pedal goes to the floor but will still stop the truck, barely. I bought an A/C Delco master cylinder hoping that “might” fix it, no luck.
Are there more components to replace?

I did another M/C earlier today on my K20, no hydro boost, no problems. Bench bled both of em. Have not bled brakes on K30 since I’m by myself.

[Dead Parrot]

Check all four brakes for fluid leaks. If it is going to the floor, sounds like the hydro boost is working fine. A similar issue on my CUCV turned out to be one of the T shaped things on a rear wheel cylinder had came out of place and let all the rear brake fluid leak out. A pair of new rear wheel cylinders + a new T thing fixed the problem. Those T things are a bit of a pain to find. Not included with the wheel cylinder kit.

[Nick_R_23]

If the brake light is coming on, it means the shuttle valve in the proportioning valve is off to one side or the other due to one side of the hydraulic system not building pressure. Your description of symptoms seems to back this up. Either your master cylinder isn’t building pressure on one port, or you have a blockage or air in a line somewhere.

[57NAPCO]

Thanks for the info y’all ! I’ll have to continue next week.

[bigdaddy4x4]

you may have the wrong master cylinder some of them where the master cylinder piston is there are differences in the hole where the rod from the hydroboost goes in one is deeper than the other i will post a pic the one on the left is for the hydroboost and the one on the right is for a 79k20 with vacum boost

[v30crewcab]

reset your proportioning valve first. get the light off then fix the leak that caused it to uncenter. you got a leak at the front or the rear.

[mongocanfly]

youll need the little centering tool, to keep the prop valve centered while bleeding the brakes

[57NAPCO]

Thanks for the great info, hopefully next week I’ll have time to pull the wheels off, I suspect the rear wheel cylinder(s)

[1774btcrew]

I literally went through the same issue at the same time on my c30. I lost the brakes about 10 miles from home. Limped it back at a low speed and once I could use neighborhoods, I used the e brake(1 hand on the handle, foot on the ebrake) Had a NOS d
Delco master cylinder for it. My original master cylinder was leaking. Bench bled and installed. Install was easy and took just a few minutes. I couldn't get them to bleed at all. Tried gravity bleeding first, then had my wife pump brakes for me. All to no avail. I went to my shop and grabbed my vacuum bleeder pump, had the wife keep Master cylinder full and pumped away. This resolved my issue. This could be done with one person but it is a lot trips from master cyliner to brakes

[57NAPCO]

Been so busy I’ve not had time to even look at the “Hero” truck, as we call him.

Thanks again for all the good info shared