Background:
This is in my motorhome with a newly swapped 93 Cummins. The hydroboost and brakes were working fine with the previous 440/727 and it's own PS pump. With the Cummins swap I connected the Cummins PS pump to the hydroboost. I also added a PS cooler in the return line to the PS pump.

Problem:
Once the engine is running (i.e. the PS pump is on) the hydroboost applies the brakes on its own power about 10% down and this turns on the brakes lights and makes for a difficult take off. The vehicle is still driveable but with felt resistance from the wheels.

The brake lights work and the wheels are free to spin when the engine is not running.

The power steering is fine and has no noticeable problems or noises.

I have bled all the air out of the PS.

Any help would be appreciated.

 

is the return line kicked or damaged?

 

Is the direction of flow correct?

Mark.

 

Seprate return lines for hydroboost and box, or tee'ed together?

 

They are teed. They were teed originally with the 440/727 drivetrain and worked fine. All I did was connect the original pressure line to the Cummins PS pump. Of course when I did that, I had to coax/bend the lines to fit; I may have overbent or kinked one or two. And I used the teed original return line to the added PS cooler, the other line from the cooler goes back to the pump.

Anyway, I'm gonna take off every hose/line tomorrow and inspect. I may even take the hydroboost apart. I hope I'm not gonna embarrass myself by finding a vacuum plug (u know those plastic thingies that we use to plug the lines temporarily to keep lines from losing fluid and/or getting water or dirt?) inside the lines.

 

I have heard of problems like yours relating to T'ing the return lines. Its works in some cases, others not. Not all Saginaw pumps have the same output, so that is probably the variable you are dealing with.

You can swap canisters on saginaw pumps - for one with 2 return fittings. Or braze in a new fitting. Or I've even seen a hole drilled in a filler cap for a return!

 

thanks for following up on this. i've thought of the difference in output of the two pumps (from gas and diesel) too. I'll try and diagnose with that info: I'll connect the return line directly to the hydroboost (leaving out the steering gear) and see if the brakes work propery then.

One other thing I didn't mention before: the original motorhome steering gear now leaks (didn't leak before) - it looks like the system has too much pressure at that side causing it to leak. What does this indicate?

 

In researching this, I found addl info from hydroboost.com forum:

Note where it says: "The installation of a cooler or a filter between the T fitting and the ps pump reservoir IS going to cause enough backpressure / restriction to flow to cause backpressure on the return line connected to the assist unit = malfunctions of mild to severe form, depending on just how much backpressure exists." I think I'll remove the cooler, it never had one before anyway, and see if the problem goes away. Or replumb/relocate the tee and the cooler lines.

"Return lines? The best answer is to either obtain a pump housing with twin return line nipples, or install a second return line nipple into your existing reservoir. If this concept doesn't exactly thrill you, the return line T fitting design can work properly instead, though it must be installed and oriented correctly, which is where some people don't pay attention / screw it up. Here's the deal: Know that you will always have a high rate of low pressure return flow from the steering box, vs the only time you have return flow out of the assist unit is when you are letting off the brakes, at which point only about a tablespoon of fluid gets exhausted out of the return line of the brake assist unit. Knowing this, you will see why it is absolutely imperative that the high rate of flow coming out of the steering box must be allowed to flow STRAIGHT THROUGH the long / straight run of the T fitting, and the perpendicular hose nipple sticking out of the right / left of the T is where the assist unit return line connects. This allows the tablespoon of ps fluid that gets exhausted out of the assist unit to simply slide in with the rest of the fluid flowing through the straight run of the T fitting. The installation of a cooler or a filter between the T fitting and the ps pump reservoir IS going to cause enough backpressure / restriction to flow to cause backpressure on the return line connected to the assist unit = malfunctions of mild to severe form, depending on just how much backpressure exists. Customers that don't follow the directions have been known to call in stating: "my brake pedal applies by itself the minute I start the engine", to where we immediately know that there is a backpressure situation, almost always caused by improper orientations of the T fitting, or caused by a restriction to flow by a cooler or filter. We then have them replumb the return line T fitting correctly, AFTER any such coolers / filters = mint. I will say that if you have installed a VERY high flow pump, that there may be simply too much flow in the return line coming out of the steering box, and it will be mandatory to install an independant second return. You will only find this with pumps flowing more than 5 GPM (gallons per minute), vs most automotive / truck applications will only flow about 3.5 - 4.5 GPM. Summary? Most of the stuff you're hearing about is because people aren't paying attention to the instructions, and overall don't understand "what makes these things tick". I will say that the most optimum way to set up the system IS to install an independant / individual return line fitting on the ps pump reservoir, though this is a task that most will not want to get into - hence the T fitting design (which can, does, and will work fine, as long as the aforementioned informations are taken into account).

Any backpressures existing on the brake assist unit return line can cause any one of a myriad of system malfunctions to occur. Given enough backpressure on the assist unit return line, the brakes may even self apply the minute you start the engine (though this would take a major backpressure condition to cause this). Note that even 2-3 psi of backpressure can upset the internal valving of the brake assist unit. Some of the lesser symptoms of backpressures in the low pressure side are excessive heat buildups in the ps system, overly aggressive brake assist characteristics, and very slow continued brake release. Do note that the installation of a second / independant return line fitting in the ps reservoir IS the best answer for the situation, though running the vastly simpler T fitting is effective in 98% or better applications."

 

Problem solved. Thanks to all who provided me food for thought. Special mention to U2Slow who pointed me to the tee issue, which gave me an additional point to research.

The solution was to remove the cooler away from the return line at the pump, thus removing that flow restriction from the booster return, i.e. having a more direct return line for the brake booster. Obviously just that small restriction shifts the balance of the steering gear and brake return pressures.

I inserted the steering cooler between the steering gear return line and the tee and all is well, brakes and steering. No more self-applying hydroboost. In short, keep the tee as close to the PS pump as possible and give the steering gear return fluid a straight line to home (the pump). The back pressure caused by the high flow of steering fluid returning and restriction in the common return line causes the self-apply problem.