RDF1

Yea your professional drawing is 100% correct. thats how this truck is plumbed as well.

RB04Av

Yup, that's essentially how mine is plumbed too; except maybe there's 2 return nipples on the reservoir, rather than a T for the 2 return lines down to 1 nipple. Can't recall for sure and I'm too lazy to go look. But that's a very minor detail in the grand scheme anyway.

In the drawing, the return from the booster to the T should be black, like the other low pressure lines. It would be clearer that way.

RDF1

I removed the hydroboost from the system and plumbed the steering rack how vacuum brakes work.
steering is still really bad.
Now I'm thinking something is wrong with the pump. Thats the last part in the system.

Anybody know what pressures should be in the hydraulic system?

RB04Av

Near zero (like, less than 100 psi) if there's no force trying to turn the steering wheel; up to around 1400 psi or so when the wheel is at full lock. Different vehicles typically call for anywhere from 1200 to 1500, not sure the exact number for these trucks, but it'll be somewhere in there for sure.

The PS system is constant volume. The pump constantly moves the same amount of fluid. The pressure is low (fluid just mostly circulates uselessly) when the assist system isn't doing anything, but skyrockets when assist is called for.

RDF1

So I finally found the fittings I needed to test pressure with. Only had a 600psi gauge so I went with it.​​​​took the pump off the truck and used a drill to spin the shaft. Only 200-210psi with my drill wide open and pressure port dead headed. went and got a pump from my junk pile beside the shop and it pegged my 600psi gauge as soon as I got the pump primed. So I went to town and got a new pump. Threw it on the truck and it works perfect now. Can steer the truck with a pinky and foot on the brakes. So that was a fun ordeal. Even tho the pump that was on there was new to start with.

MikeGyver

iTrader: (5)

Name the parts store where the original bad new (rebuilt?) pump was from.

RDF1

The guy had already put 2 pumps on from Napa and they were new not remans.

Fullpower

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Made in CHINA, Quality Control not included.

RDF1

Yea it was a pain to figure all that out.

Truck now has a transbrake 4l80 so no more brake boosting.

chris99gmc

Are the PS pumps that you are buying specifically made for a Hydroboost truck? They are all the same pump externally so its easy for one to make the mistake and grab one thats valved for a non hydro truck instead.

If your pumps and rack are working fine at first, then going bad in short order, you likely have a leak somewhere. You would probably notice the PS pump whine though, if this is the case.

On the other hand, from reading through the thread, it sounds like an output pressure issue from the pump. i ALWAYS make the habit of removing the internal valve inside of these pumps to inspect the number of shims on the pressure relief valve bolt(there are thin shims or locktite under the nut with the screen on the end). This is what ultimately sets the output pressure of the pump. Just don’t mistake pressure with flow, as they are two different things.

as long as you have good quality brake lines and fresh new fittings on your brake calipers, I would personally pull out all the shims from the relief valve. Then snug it back down flush with the valve. If you are getting pumps with a worn out bore, fluid can seep past the valve and cause pressure loss. Take note of the clearance the valve has in the bore. Also note if its an aluminum body pump with steel valve or vice versa. Aluminum expands more w/ heat than steel does. So clearances can change as the pump and fluid heats up.

Also keep in mind that more output pressure assist can cause unwarranted leaks on the other end. This may Usually show up with the brake fluid seeping its way out of worn internally rusted calipers and bleeders. I had to ho back and re seal the threads on the bleeders of every hydroboost upgaded truck i’ve done. Might be a good opportunity to upgrade to some stainless lines and all new crush washers and bleeders if you end up going this route.


i also want to add that it’s not a good idea to spin these pumps with a drill. Most drills these days have an internal brake or chuck that slows them down when you release the switch. This will abruptly stop the pump from spinning fast enough for the shaft to start breaking its way loose inside the pump. It may not show up right away, but all it needs is once or twice to break its specific torque. Then in time, it works its way loose and leaves you stranded on the side of the road with a thrown belt and a mess of ps fluid.