W123 Weak Brake Pedal
 

Seems my brake pedal is harder than usual and not very strong. However, when cruising on the highway and stepping on both the throttle and brake, the brakes are quite strong feeling.

Obviously, vacuum related somehow. How much vacuum is the vac pump supposed to output (assuming isolating measurement to just the pump itself)??? How much vac is the booster supposed to get??

What's the easiest way to test the brake booster for a vac leak? It's a big object that would probably take my Mityvac forever to suck down. Maybe disconnect the hardline, then clamp on a hose to the nipple and vac it down with a Mityvac?

I think you're looking for 21-23" Hg at the pump. If the vacuum pump is failing, eventually that brake pedal turns into a cinderblock that doesn't do anything and you can die.

Sometimes it's just the check valve at the pump inlet.

http://www.dieselgiant.com/mercedesvacuumtroubleshooting.htm

I wouldn't 100% conclude its vacuum related. Maybe bleed your brakes after u check the vacuum pressure

You can measure vacuum directly at the vac pump with nothing else in the circuit at all. If the pump tests good (18-20"), add things in and see that vacuum doesn't drop off from a leak. You can connect a MitiVac directly to the brake booster and it should hold. It may take some pumping because the booster is large compared to the MitiVac but it will pump up. Make sure any leakage isn't due to faulty connections or lines going to the test equipment.

I am 100% sure that a hard pedal is not a symptom of brakes that need bleeding.

Seems my brake pedal is harder than usual and not very strong. However, when cruising on the highway and stepping on both the throttle and brake, the brakes are quite strong feeling.

The vacuum pump or check valve is going. If this is your get to work car, you don't have much time to fix it before it can't be driven safely.

Seems this is afflicted by check valve failure at both the pump and on the plastic hardline to the brake booster. That'd explain why the brakes work great when on the throttle and for a moment after i let off when on the highway. When running the motor at 3500rpm, the vac pump making plenty of vac, but soon after letting off the throttle, the vac leaks/disperses out to the rest of the system.

Fun facts:
Vac pumps makes about 25 mmhg at the brake booster (when blocking everything else off)
Neither side of the plastic hardline that goes to the brake booster holds pressure - Check valve failure.
Brake booster holds 20mmhg
Metal hardline that extends from the vac pump does not hold vac. It tries, but leaks out after a few sec.

Is it normal for the check valve on the vac pump to "sorta" work??? If it worked perfectly, then i'd have a heck of a lot more vac at my booster. But.... the check valve in the plastic hardline is almost redundant...

It's common for the retainer to break and allow the spring/plunger to escape into the pump inlet, thereby blocking air flow. The pump itself will work fine without that check valve. I suspect that the valve exists primarily to prevent engine fumes from being sucked into the system when the pump is not operating. Removing and inspecting the pump inlet valve might be a good idea.

I think i resolved the vacuum pump check valve issue. Took it off and cleaned it with WD 40. Seems to hold now. However, I think effectively testing the valve's action is rather difficult. Wouldn't be surprised if my testing apparatus was leaking. (Had to switch to a bigger conical fitting to shove into the metal vac hard line)

Is this check valve supposed to be sealed to the pump with a copper or alu crush gasket?? The valve has a circular ridge for one, just like a drain plug...

No.