Hydraulic system problems ('85 5000 turbo)

E. Roy Wendell IV erwendell at mac.com
Thu Aug 17 22:47:17 EDT 2006

> Ameer;
> You may have a bad brake servo. When you run the engine, the brake  
> bomb is
> charged and the reservoir level goes down. When you stop the  
> engine, the
> pressurized bomb discharges through the leaking servo into the  
> reservoir and
> the level goes up. A badly leaking servo will prevent enough  
> pressure from
> building up in the system to provide brake boost - the fluid  
> recirculates
> from the pump through the leaking servo back into the reservoir.
> There is a standard test for the servo, but the test is not  
> effective if it
> is leaking so badly that the bomb discharges rapidly . To test a  
> servo with
> a bad leak, remove the return hose from the servo to the reservoir  
> and pinch
> it closed. Attach a hose from the servo return and lead it into a  
> bottle.
> Start the engine. DO NOT APPLY THE BRAKES! If the servo is bad, oil  
> will be
> flowing into the bottle. If it is good, no oil will be coming out  
> of the
> hose. NOTE: Make SURE you remove the RETURN hose, NOT the FEED hose  
> from the
> bomb!
> This is based on my fading recollection of the Type 44 hydraulics.  
> I just
> went through this with my UrS6 and FWIR the systems are fairly  
> similar in
> function.
> Fred Munro
> '94 S4
> '97 S6

I agree with Fred and disagree with a previous poster who said that  
the fluid level should change running/not running. When the system is  
operating correctly any fluid that leaves the reservoir goes to the  
pump and then returns to the reservoir via either the brake or  
steering system. If the pressure relief valve in the bomb is working  
and the brake booster is good then the bomb stays filled all the  
time. The only time that there will be a large change in fluid level  
is when the bomb fills. This would normally only take place when a  
new bomb is installed or the bomb had been completely discharged by  
pumping the brake pedal repeatedly with the engine off.

A quick rundown on how the type 44 hydraulic system works:

The pump is actually two pumps in one. The forward section (closest  
to the pulley) is the high pressure low volume pump for the brakes  
and contains two pistons. The middle section is the low pressure high  
volume system for the steering and contains six pistons. The pistons  
are all run by a common eccentric lobe on the pump shaft. Fluid for  
both systems is supplied by the middle large line from the reservoir.  
Fluid enters the middle of the pump body and is then transferred by  
the pistons to the area underneath each one of the associated plus  
shaped caps. For the brake system, the fluid flows from the area  
under the two caps to the front hard line that leads to the bomb. For  
the steering system, the fluid flows from each cap back to what is  
essentially a reservoir in the back third of the pump housing. In  
that reservoir is a pressure relief valve that maintains steering  
system pressure by allowing excess fluid to return back to the intake  
side of the pistons.

When the spool valve in the rack is displaced by steering force,  
fluid flows from the pump via the rear line, through the rack, and  
back to the reservoir. That's it for steering.

The brake system flows continuously from the pump to the bomb. There  
is a pressure relief valve in the bomb that sends excess fluid back  
to the reservoir and maintains system pressure. This excess fluid  
travels via the low pressure line that leads from the aft end of the  
bomb to the bottom of the reservoir. When brake pedal force is  
applied to the booster, its internal valve opens and fluid flows from  
the bomb to the booster via the small metal line that exits the side  
of the booster. When the brake pedal is released, the fluid that  
entered the booster then returns to the reservoir via the combination  
metal and rubber line that goes tangentially into the top of it. The  
bomb serves to store fluid under pressure in the event that brake  
assist is needed with the engine not running. It does this by way of  
nitrogen gas under high pressure pressing on a rubber diaphragm. When  
pump pressure is present, the diaphragm displaces in the direction of  
the gas and therefore there is volume for the fluid to fill. When the  
pump stops, the pressure relief valve closes and holds fluid in the  
bomb. If the brake pedal is pushed, the nitrogen gas pushing against  
the diaphragm provides pressure to operate the brake booster as  
normal until there is no fluid left in the bomb.

Whew, something tells me I should apply for a tech writer job somewhere!

E. Roy Wendell IV
erwendell at mac.com
Too many type 44 tq
A pair of MR2s

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