Hydroboost Braking System

[Rickf1985]

Well, it is 20 degrees out with a 40 MPH wind so I am not going out to look at my Jeep which I have put Hydroboost on but it is all out in the open so I can wrap my mind around the lines. The pressure line feeds the power steering and then goes from the steering to the brakes and then from the brakes to the return back to the pump. When using the steering the return from the steering use goes through the steering return to the pump. I am thinking there is a restriction in the return of the hydroboost that is causing a pressure back feed through the whole system. It may not be in the return line because that line is only held on with a hose clamp so if you put 1,000 lbs. of pressure on it then it will definitely pop off. I suspect there may be an issue in the hydroboost unit itself. Wait and see if Dave chimes in on this since he also has a lot of brake experience. The hydroboost replacements are not outrageously expensive but I don't feel perfectly confident to say "yes, THAT is the problem".

[DaveVA78Chieftain]

As far as hose connection are concerned on a P30:
Pressure side: From pump to hydroboost to steering gearbox
Return side:  While the picture above shows a "T" connector, there are separate return lines to the pump from steering gear and hydroboost unit on the P30 installation.

You need to use the power steering hose drawing (pdf page 75; page 6-3) in the Chevrolet Motor Home Chassis Service Guide and the power steering hose drawing (pdf page 214; page 3B3-18) of the 1980 Light Duty Truck Service manual to see the hose routing.  Please note that a 454 uses the LE8 designation (Fig 3B3-42; P300 (32)).  The P300 (42) chassis is the commercial chassis (breadtruck).

From page 3-5 of Chevrolet Motor Home Chassis Service Guide
HARD STEERING AT ENGINE IDLE
The P-Series motor home power steering assist system is designed for good response up to the 5,000-Ib. suspension capacity. Complaints of little or no steering assist while at idle or with the driver's foot applying the brake are usually the result of the suspension being at or very near capacity. The reason this occurs is that the Hydro-Boost system has taken some power away from the steering assist system. At this point, the power steering assist system is at borderline capacity.

This situation can be corrected by removing the pressure on the brake. This will return the necessary pressure to the steering assist system allowing proper power steering assistance. Also, if additional weight were added to the front axle there would be a momentary system stall requiring slight movement of the vehicle in order to "feel" the power steering assist system operating . GM cannot endorse overloading . To avoid problems concerning overloading, move some load rearward to remove some of the weight from the front axle. Axle weight should never exceed tire or axle capacity. Refer to the proper shop manual for the power steering pump pressure checking procedures.

NOTE :
Typical P-Series - Commercial power steering pump pressure is 1,200-1,300 PSI.
Typical P-Series - Motor home power steering pump pressure is 1,350-1,450 PSI.
In order to improve static steer effort on the P30 (motor home chassis, the steering gear has been changed on all 14,500) - 14,800-16,000 GVW chassis effective on V.I.N. N3310596. The new 710 gear ratio is 17.5:1 and the older 708 gear was 14:1 . The new 710 gear cannot be installed on the older units due to numerous changes and supports.

So my first question is, how close are you to being overloaded?
If not,
Then the spool valve in the hydroboost unit maybe sticking
Hydro-Boost Brake Systems
or the pump may be giving up the ghost.

Concentrate on one circuit at a time:
Brake OK, Steering problem
Brake problem, steering OK
Both brake and steering problem (pump or hydroboost high pressure flow problem)

[cook elandan]

Rick/Dave, maybe what i should have asked first was, is there any brake pedal movement while steering. When I bought it I did not pay any attention to the pedal when I had been driving.

[Rickf1985]

No pedal movement when steering. Now, if you are in a turn and stomp the brakes you will feel it in the steering wheel as a small jerk. It will not make you go one way or the other, it is just a sensation. I just went back and reread your original post since you ask this question. I am now beginning to think it may be the steering return that is blocked since you say you cannot turn the wheel very far before it stops. That may be feeding back pressure into the brakes OR, and I didn't think of this til now, there could be a restriction problem in the return side of the pump feeding pressure back the line of the opposing unit. The only way I know of to properly diagnose this is with pressure gauges. If you want to try to do it without it is going to get messy and semi-complicated. You will need to take the pressure line from the steering to the booster and disconnect it and connect it directly to the booster return to the pump. Basically you are taking the booster out of the circuit. Try the steering and see if it works. If it does then I would swap the return hoses on the pump and see what happens. If one is blocked it may blow one off so this is where it would be messy, have lots of speedy dry on hand. I don't think it is in the pump though, I am leaning towards the booster. The last thinking I am thinking is a blocked pressure port in the booster that would prevent fluid from the steering from getting from the steering box into the booster. That may cause your problem. I have never had that happen so I honestly do not know but I do know that the fluid has to go somewhere or things stop. This is all looking like a booster but I am not confident enough to say go buy one without personally seeing pressure gauges.

As much as I try to help people do things at home themselves this is one that you may want to consider taking to a shop unless you have the tools at home.

[DaveVA78Chieftain]

Sorry Rick but my research says for a stock P30 configuration:

Pressure side: From steering pump to hydro-booster to steering box.

Return side: Separate return hoses from each device back to steering pump

So, a blockage in the booster side can prevent pressurized fluid reaching the steering box.

To split the circuit, you will have to disconnect the pressure hose from the steering pump to the booster and connect it to the pressure input port on the steering box.  I have no clue if the fittings are the same.

Please note I do not own a P30 rig so if anyone can confirm what I just said about hose routing it would be appreciated.  Iused several different years of service manuals to deduce this.

From page 1 of this Hydroboost thread:

HYDROBOOST OPERATION

RELEASED POSITION (NO BRAKING)
ln this position, spool valve return spring holds spool valve open. In open position, spool valve provides unrestricted fluid flow between power steering pump and power steering gear. Fluid pressure is blocked from entering boost pressure chamber by lands on spool valve. As fluid pressure increases with steering demand, it has no effect on boost pressure chamber. Boost pressure chamber is vented through spool valve, to pump return port, and back to power steering pump.

BRAKING POSITION
As brake pedal is depressed, it moves pedal rod and initiates movement of spool valve. This closes fluid return port to pump from boost chamber, and admits fluid into boost chamber from pressure port. Additional valve movement restricts flow between pump and steering gear, causing pump to increase fluid pressure to maintain flow rate to steering gear. As fluid pressure increases in boost chamber, it forces piston forward actuating master cylinder piston, resulting in brake application. lf fluid pressure is required for steering while braking, pump pressure will rise and spool valve will shift in an open direction allowing more fluid to flow to steering gear.

[Rickf1985]

Thanks Dave, That is why I was hoping you would chime in. I am going to go look at my Jeep setup tomorrow and try to think of some scenarios but really this is a problem that needs to be checked with the proper gauges. You are working with between 1,000 and 1,300 lbs. of hydraulic pressure and you do not want that blowing up in your face.

[DaveVA78Chieftain]

Regarding the flow of pressurized fluid from the pump through the hydroboost unit to the steering gear, here is a good description of how it flows through the hydroboost unit http://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/
It also has a lot of troubleshooting information.

[cook elandan]

Rick / Dave, Thank you for the time in answering the questions.  The last post from Dave was great.  I will start with the return line. Then check brake fluid movement, Then flush and retest.  After I get this taken care of, I will start on the great "why my electric fuel pump is not working".  Having fun as always.
Thanks,
Guys

[rvers]

HELLO!
I have a 1990 coachmen SB270 A class on a chevy chassis.49k miles on the old 454 with the 3 speed transmission. TBI gets me about 10 mpg, not so bad for hauling 5 plus tons down the road, all I need to do is stop 5 plus tons.
This is where my current set of issues are. I replaced the power booster(brand new unit) and did not install the push rod the first time(my bad but have it installed now). I filled up the reservoir, cranked it a few times, check the level it was to much so I removed some of it to the proper level. Then I started it and now the brake pedal goes down to the floor(almost all the way) when the motor runs, I shut off the motor and the brake pedal returns to the full unladen position. I tried to bleed the air out of the system by pumping the brake pedal 4-5 times with the engine off, still no change(pedal to the floor with motor running.
So If i may ask a greenhorn question, and that is ,what is causing this to happen? My thoughts are that there is excessive pressure/vacuum causing the pedal to go to the floor w/motor running.
No leaks of any kind but I did notice that the booster/steering pump gets very hot after about 3 min run time.
Any ideas or thoughts would be very helpful.
I would like to thank you in advance for any and all help!
We are trying to get to upstate New York next week and this kinda puts things on hold till the issues is resolved.
Cheers
rvers

[Rickf1985]

All the booster does is adds power to the rod going to the master cylinder. If the pedal is going to the floor then either the rod you put in is wrong or the master cylinder is bad or there is no fluid in your brake lines. The pedal should be hard to push once you bleed off the accumulator charge which take about three pushes of the pedal under normal use. with no power the pedal should be hard and high but if you can still push it to the floor with some effort then the rod or main brake system is at fault, not the booster. There is a mechanical connection from the pedal to the master cylinder is case of booster failure.

[rvers]

All the booster does is adds power to the rod going to the master cylinder. ok got it now what part of the system tells it to depress or return? Is that the power valve?


I used the old push rod as per the manufactures install data.
My pedal bleeds off fine and get hard and high with engine off.
something it telling the booster to apply pressure to the rod(power valve?) when the engine is running and not allowing it to return to normal operating conditions is my thinking as of now.
I have read the 3 pages here on the hydro boost issues and so far nobody has the pedal to the floor with engine running issue.
Can a loose belt or a blocked return line cause this issue? I will be checking them tomorrow.
I would now like to thank you for both your time and knowledge Rick and perhaps we can crack this nut together( with a little help from our friends here)
Cheers
rvers

[DaveVA78Chieftain]

And here is Hydroboost information: http://dave78chieftain.com/hydroboost_brakes.html

[rvers]

Dave, Nice name, as it turns out I too am a "Dave" lol.
Thank you for correcting my post    not sure what happened there
Thanks for the info    and the links but as it is they are not helpful at all regarding my issues. I have read and read and read and still there is no mention of the brake pedal moving towards the floor(moves about 1/2-3/4 of the way down when the engine is started). All fluids are full and new. I installed a brand new hydro unit,filled the steering fluid, bleed the lines of air by turning the wheel. I also released the reserve pressure via the pedal with the engine off. I have tried everything mention in the posts and still no change in the pedal movement.
Now if I may ask the following questions please 
1, What provides the power to the spool valve to operate it?
2. What then tells the spool valve to apply pressure to the brake push rod?
3, What then tell the spool valve to release the brake pressure?
For some reason the brakes are being applied all the way when the engine is started via the spool valve,push rod. So I am thinking now that perhaps I got a bad unit and need to return it?
Have a great day folks!
Cheers
rvers

[DaveVA78Chieftain]

Does this help?
http://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/

Best I can offer this morning.  Off to work - Later

[Rickf1985]

Sounds to me like you have a brake system issue not a booster issue., it is pushing the master cylinder all the way down which means the problem is beyond the booster. Look in the top of the master cylinder if you can while pushing the pedal down and if the fluid is moving around then the booster is working fine, the problem is in the brakes.

[DaveVA78Chieftain]

I have read and read and read and still there is no mention of the brake pedal moving towards the floor(moves about 1/2-3/4 of the way down when the engine is started).

Please read the HYDROBOOST FUNCTIONAL TEST section (top left) on page 10-24 (pdf page 2) of Hydroboost Description and Repair (was a link at the bottom of my page).  If that test passes (no fluid in accumulator), then I think you are OK.  I suspect the drop your experiencing with accumulator full is the same thing.  In other words "The sinking of the pedal when the engine was started is a result of the power chamber being pressurized." as described in the Pedal Feel section of http://www.brakeandfrontend.com/operation-diagnosis-and-repair-of-hydro-boost-power-assist-systems/

[rvers]

Dave with all do respect i have read and tested all that I could. I now know how it all works and have come to the conclusion that the unit itself is faulty. I have great fluid flow from the pump steering box and the return line. I pulled the return line and found it flowing out with fluid from the pump. I then started the engine and there was fluid coming out of the return port. It has to be a stuck spool valve that is only letting the fluid to activate the push rod and not allowing it to power chamber to depressurize(weak return spring is my call), that does happen(pedal comes up) when I shut off the motor tho. The tolerances are so close between the spool valve and the chamber that a weak spring may be all it takes
I removed the MC, all full with fluid and I do have fluid movement while pressing the pedal with the engine off. no signs of any leaks in the lines or at the valves I also have the power reserve working as per the three pushes of the pedal with each push requiring more pressure and less movement. I am pulling the unit tomorrow morning and returning it for another one. I will update ASAP.
Again I do appreciate all you time and efforts and most of all your patience in dealing with a greenhorn!!!
THANK YOU!
Cheers
rvers

[DaveVA78Chieftain]

I understand what you are saying.  Hope the new unit solves the problem.

[Rickf1985]

Hydroboost can be a pain, I am going through that right now with my 98 Dodge diesel dually. Slow pedal return was hanging the front brakes on and smoked the pads on a busy interstate with a full load and full trailer. Not fun. On my second unit now with same symptoms, they can be frustrating.

[bnlfan]

3/3/18
Can we update the photo's....thanks

[arjen]

Since we are at it,


I own a 1975 2900 chevy sportscoach.
The hydroboost is leaking from the front,
Pipes and connections are dry.
Can't find any replacement or rebuild.
Any suggestions?
Thx

[Oz]

Give Geoff at Alretta Truck Parts a call.

http://www.alretta.com/

[arjen]

Thx


Will send them a mail!!!

[DaveVA78Chieftain]

Cardone Part # 52-7249

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