Hydroboost issues
 

I am running a disc brake conversion front and back on a 70 C20 4x4. I am just getting into the hydroboost troubleshooting. My lines are layed out as per diagram shown, and I'm using a disc/disc master cylinder. What's happening is, power steering is working fine, air is bled, the brakes work but pedal goes to floor then pushes back at me, and when stepping on the pedal I feel the steering wheel move slightly. Also when engine is off pedal feels fine. And yes brakes are bled
Any thoughts on this ?

https://beamingpix.com/images/2025/02/02/1000003850.jpg

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https://beamingpix.com/images/2025/02/02/1000003859.jpg

Re: Hydroboost issues
 

Make sure the rod is adjusted perfectly inside cab

Re: Hydroboost issues
 

What would be the procedure for that ?
The pedal feels normal with engine off

Re: Hydroboost issues
 

Have you tried taking the return line from the hydroboost and just putting it inside the reservoir instead of inline? I have heard that returning it like you have it can cause problems, although some people do it that way.

Re: Hydroboost issues
 

Some people use the "T" in the return without issues and others seem to have issues.
I put an extra return in my reservoir just to be sure.
Also, is it a new hydro-boost unit or a used one? I had one on a truck once that was faulty.
It started by kicking back on the pedal like you describe, and then it self-applied the brakes and would not release. I had to replace it.

Re: Hydroboost issues
 

I used a tee on mine, and have had no issues. The first square body trucks that had hydraboost from the factory (around ‘77 or so) had the tee from the factory fyi. I’m running a 72 pump and gear with an 09ish hydraboost from a Chevy 2500 hd gas truck. Are you still running all drums beings it’s a ‘70? I assume not because you said you have a disk/disk proportioning valve. If not did you remove the residual pressure valves from the lines down near the wheel rubber hoses? Can you give us a break down of what exactly you have as far as brake components go? The only thing I’ve run into on my truck is I have to fill the reservoir with it running because it doesn’t hold enough to fill the system with the OE pump/reservoir setup. It will start cavitating and foaming the fluid really bad if you don’t fill with it running, this is from completely empty. Topping off is fine with the engine off. How did you bleed the system?

The pedal pushing back at you will happen when I goes to the floor. My 98 K2500 did this when I blew a brake line and lost the pedal.Turning to the left when the pedal bounces like that is has happened to me in the same truck, not enough to pull you into another lane on the road just enough to notice in the steering wheel if that makes sense. Are your rubber brake hoses new? I suspect you might have an issue on the hydraulic side of the system.

Re: Hydroboost issues
 

This is a complete frame off restoration. 4 wheel disc brake conversion from lugnut 4x4, I believe hydroboost is from a astro van but can't confirm. All new brake lines, disc/disc master cylinder which I'm changing to drum/drum as per lugnut 4x4 advice ( their system is designed to be used with stock master cylinder) brakes where all bled properly. Hydroboost was bled by turning the wheel 1000 times and depressing brake pedal until the bubbles disappeared , the pedal feels normal until I start truck
https://beamingpix.com/images/2023/0...y_1024x768.jpg

Re: Hydroboost issues
 

That makes perfect sense if they planned the system around the drum master, your symptoms definitely sound like a hydraulic issue like I said above. Report back when you swap it out please. I’m curious what you find.

Re: Hydroboost issues
 

Wouldn’t a disc/drum have a residual valve for the rear drums whereas a disc/disc would not ? I had to change the master in my Chevelle when I put rear discs on as the rear .

Re: Hydroboost issues
 

Is your hydroboost new? I went with a used one, and had to troubleshoot issues. I had the same symptom your describing, but probably worse. It was a check valve seal within the hydroboost that was causing my issues. It uses a plastic seat at the bottom of the bore that needed to be removed and replaced.

Re: Hydroboost issues
 

A few suggestions-

If you used a junkyard Hydroboost, get all the seals, spool valve and check valves working correctly with an overhaul kit from Pirate Jacks. This solved a few issues I had with a junkyard Astrovan Hydroboost.

The clearance of the actuator pin of the Hydroboost to the master cylinder cup that receives the pin is critical. .020" IIRC. You can buy a tool from Lisle to do the measurement or just use a dial caliper and a straigt-edge. They sell throw-away 3D-printed ones on ebay too

A power brake system with a hydroboost can be difficult to bleed. I've had some luck with a Mighty-vac pump to put some vacuum in the system. I put it under vacuum with the front wheels jacked up and turn the wheels lock to lock a few times, Repeat with engine running.


I've run a dedicated return to the reservoir and a "t" in the rack return line and haven't noted any difference, but some folks say they have. My latest pump is a Saginaw 'ham can' and I welded a -6AN fitting about an inch below the fluid level

https://talk.classicparts.com/media/...ring.7194/full

When working correctly the hydroboost on my truck is fantastic, Hope this helps you some