Aeration in P/S-Hydroboost Fluid

[rustyk]

My motorhome is a bit of an oddball - 28' diesel pusher with the 6.5L TD and Hydroboost. The supply and return lines from the P/S pump to the Hydroboost brake booster are about 30' long. For the past two months, I've suffered aeration in the system, to the point when I shut the engine down, the reservoir spouts 3-6 oz. onto the ground.

I've replaced the Usual Suspects (P/S pump and brake booster) but the issue persists. In addition to the power steering and brakes, the system powers the shaft brake.

Any hints as to the source of the aeration would be appreciated.

[DmaxMaverick]

That's gonna be a tough one, absent an external leak. Check any metal supply/return lines near any heat source (like exhaust pipe, pusher engine or generator), or a section subject to vibration (like a loose/missing retainer or clamp). The supply is high pressure, and the return is low pressure (not suction), so that makes it difficult to pin down. If your reservoir is only the little tea cup mounted to the pump, perhaps you need to install an external/isolated reservoir (at least a quart or two, with an equal volume to allow for expansion), like the early 6.2's and HD GM trucks had. This eliminates aeration, and it seems your application would require the additional volume (at least).

A Possible answer to your problem is the volume itself. ATF/Hydraulic fluid expands/contracts significantly at the temp swings it will see, and your system will have a greatly exaggerated temp swing, compared to "normal" applications, such as in a truck. The long lines, in itself, is a cooling system. Your system will have at least 6 temperature/friction zones, each playing a part in the fluid real volume and viscosity.

[rustyk]

Thanks for the help. I'm going to inspect/replace the return lines when the weather dries up.

It appears the ATF is leaking back from the brake booster, as the reservoir is full when the engine's started and off the dipstick when running. Thake 4-6 oz to bring it back visible.

[rustyk]

I forgot to mention the reservoir is remote, between pint and a qt. capacity.

Temp was measured after a 40-mile run and was ~105° F at the return.

[rustyk]

The problem has been isolated, and won't be of interest to most forum members: Defective hydraulic actuator for the shaft (parking) brake seems to be the problem.

[DmaxMaverick]

Details, please. Inquiring minds want to know......

[rustyk]

Because of the weight of the M/H (16K GVW), the Park pawl on the 4L80E wasn't considered strong enough for the weight (and there is no hand/demergency brake).

The Hydroboost system on my coach is more complex than most. The P/S pump supplies the Hydroboost, then the power steering, then (as needed) pressure to the parking (shaft) brake actuator. When the coach is in "park" the pressure is vented from the actuator and spring tension clamps the brake. When in any other position, the supply valve opens, pressurizes the actuator, which tensions the cable to the shaft brake shoes, and retracts them.

Placing the selector in "park" vents the actuator directly into the remote reservoir.

Anyway, I had the actuator replaced today, and the problems seems to be fixed. I won't know for sure until I got through the Rite of Bleeding - again (there's a lot of hose adn places for air to collect).

[rustyk]

Final Assessment:

The parking brake actuator has a diaphragm, and mine leaked, so even though the P/S reservoir was full, the canister filled up to where the pump was sucking air due to the volume injected into the canister.

When stopped, the aerated fluid blew back and "releived itself" by overflowing.

[Robyn]

Rusty

Glad you got the thing fixed

Sounds like your brake system works similar to the MAXI brakes on a big rig. (air compresses large coil springs in the canister and releases the brakes and when the air is removed the spring applies the brakes)

Never seen one of those type systems before, must be something common to motor homes and business class diesel powered rigs.


Robyn

[rustyk]

Nope, Missy, no air except for the horns. The parking brake actuator stays dormant in Park, and the springs on the brake itself keep it engaged.

When the shifter goes into any other position, the intake valve to the actuator opens, and it pulls (by a choke-type cable) the brake into the disengaged position.

If the actuator diaphragm leaks, the entire cylinder fills up, and the P/S reservoir doesn't have enough capacity to keep the P/S pump filled after the actuator has drained so much.