Rear brake lockup after hydroboost install

[Hertzdaddy]

I don't know if its related or not but my rear drums on my 73 Jimmy are locking up very easily after doing a hydroboost conversion (from vacuum assist.) The hydroboost system was from a 79 diesel and works well for the front discs. Was there any difference in the prop valve for the hydroboost system? Any other possibilities that might cause this? I didn't change anything else with regard to alignment or suspension adjustments.

BTW, I switched the system to hydroboost due to the lack of vacuum from my choice of cam.

[Special-Ed]

sounds like you need a new proportion valve

[knomadd]

If you changed the prop valve when you switched to hydroboost, you might have a disc/disc prop valve instead of disc/drum. Regardless of what you have, you can put in an adjustable prop valve and dial it down for the rears. You've introduced a whole hell-of-a-lot more pressure to the system than the vacuum booster so your new system is overpowering the rears. I added a hydroboost system to both my classics with disc/drum setup and disc/drum prop valves with no lock up issues.

[Hertzdaddy]

I didn't swap out the prop valve on the hydroboost install but the build tag that came with the truck shows it was a disc/drum to start with. Hmmm.

[wraprail]

So i'm not getting this whole hydro-boost thing. Lets say your braking system on your Blazer takes X amount of pressure to lock up the brakes with the factory installed vacuum booster. Now you switch over to a HydroBoost with no other changes. Wouldn't it still take the same pressure to lock the brakes?
Do they use a different size master cylinder (bore size) with the HydroBoost?
Did you use your old master or from the doner?

Not sure how the HydroBoost can overpower the brakes.
Did the build tag show the brake code?
Which master you use is important.
I looked around some and with the HydroBoost there are several masters depending on
the brake code. I could not find a blazer with Diesel till 1982 and that year lists a couple different master cylinders depending on that brake code.

[Hertzdaddy]

Same master as was originally on the Jimmy when I got it. I just swapped boosters. The hydroboost came from a diesel truck.

The brakes are a lot touchier so maybe I'm getting into lockup stage easier than before. It was an emergency stop that caused it. I just don't ever remember locking the rears with the old system.

I went back and double-checked the ID (glovebox) tag and there is no mention of disc/drum or brake code. My memory isn't so good anymore I guess.

[wraprail]

I think I would get a master for a Jimmy with a diesel/hydroboost. You would have to get one for a 1982 or newer Jimmy. I checked at RockAuto and they show several different masters. I suspect the master and/or the combination/prop valve could be your problem. I would start with the master.
The HydroBoost may require a master with a larger bore size than a vacuum boosted system.

A larger bore size = lower pressure.

[dennislbrooks]

I used an 82 Caprice diesel hydroboost and pump. I used a corvette master cylinder 75 or 76 I think. Then I used my stock splitter. I added an adjustable pressure valve in my rear brake lines with the round adjusting knob under my dash to adjust the ar. I have disc all the way around now but when I first went to hydroboost, I had drums on the rear.

[knomadd]

Hydroboost will not change how your brakes operate. They will lock at the same pressure with either booster. A hydroboost simply reaches that pressure sooner in the pedal travel and can achieve higher pressures than a vacuum booster. If the booster is the only thing that changed and an emergency stop situation is what caused the brakes to lock up, not during normal driving, then everything should be working correctly (assuming everything worked correctly prior to the change to hydroboost). If you're getting brake lock up in normal driving situations then either you're pressing too hard (a hydroboost takes less foot pressure to achieve the same brake pressure as vacuum) or you need to add an adjustable proportioning valve between the master and rear brakes to reduce the amount of pressure delivered to the rear.

With hydroboost in my GTO and my C10, the brake response is much like a new vehicle. It doesn't take a lot of force from my right leg to make either stop. And they both stop sooner than they ever did with a vacuum booster. Both vehicles are disc/drum and I've never locked up either one. But I've never tried or had to make an emergency stop.