Community Forums

PapaGrimes · 03-25-2025, 05:48 PM

My Freightliner MC (FRED) has an Electric Vacuum Pump for the brakes/master cylinder. See the attached picture for details!

I get the vacuum reserve chamber and the vacuum pump!

I don't get the extra reservoir on the vacuum pump exhaust! It was half full of muck which I think was originally windshield washer fluid.

Questions:
1) Why would a Vacuum Pump need an extra reservoir?
2) What liquid should it contain?
3) How full should it be?

As always, I appreciate your help and comments!

SafariBen · 03-25-2025, 08:15 PM

1. it doesn't need an extra reservoir on the outlet
2. no fluid

That hose is the only part that doesn't look factory. The only possible reason to have an extra reservoir would be a muffler. Are you sure the vacuum is for brakes? usually brakes are only run off an engine driven vacuum pump (if not manifold vacuum) and that sort of vacuum pump is only for the HVAC controls.

zrock2010 · 03-28-2025, 06:49 AM

ya that is not factory. I doubt that pump is for the brakes as it would not be able to supply enough vacuum. Like said above its probably for the HVAC system

wolfe10 · 03-28-2025, 07:03 AM

Quote:

Originally Posted by zrock2010

ya that is not factory. I doubt that pump is for the brakes as it would not be able to supply enough vacuum. Like said above its probably for the HVAC system

Correct. That vacuum operates the dash HVAC system.

Diesels do not generate vacuum. You will find the same vacuum pump on Ford diesel PU's, etc

Skip426 · 04-03-2025, 09:04 PM

Someone has attached the hose with the clamp in error !

The tube out of the pump is an exhaust for the air the vacuum pump pulls out of the HVAC control system and at most would have a small foam filter , to keep out dust when the pump isn't running.

Can you post a picture of your " master cylinder "

.

I don't have any info on the vintage FRED chassis .

PapaGrimes · 04-04-2025, 07:22 PM

Master Cylinder and Booster.

Part Part No.
Master Cylinder 2239045
Hydro-Max Brake Booster 2772114

SafariBen · 04-04-2025, 08:58 PM

Bosch? hydromax, run by the power steering pump hydraulics, the electric motor on the bottom provides emergency braking boost in case of hydraulic failure, be sure that motor runs and provides some pedal boost when the engine is off.

Skip426 · 04-06-2025, 11:47 AM

As Ben has mentioned ; your brake boost is provided by hydraulic pressure from the power steering pump .

To test the electric pump ( back up brake assist system ) ; with the ignition key off . pump the brake pedal till it feels solid , then turn on the key without starting the engine .
Working correctly the electric motor/pump should build brake assist pressure in 15/20 seconds , and the brake pedal should feel the same as it does when the engine is running .

SafariBen · 04-06-2025, 07:33 PM

The two I've driven, the pump comes on anytime the brake pedal is depressed, and there is no hydraulic pressure (engine off basically). So before you put the key in, press the brake pedal, initially it may be stiff but once you move the pedal, you should hear the electric motor and feel the pressure drop from the brake pedal so it goes down a bit. It WON'T be as soft as with the engine running, but a lot softer than if you pull the battery cable off or the pump is not working.