Brakes Weak
 

Hello I have a 79 300D Non-Turbo that lost vacuum and pushed oil
out the exhaust tube into the air cleaner a couple of weeks ago.
It had a hole in the diaphragm.

I have replaced the pump with a rebuilt one and it seems to be working
ok. It makes around 22 on the vac gauge. I replaced the master cylinder
and I think I have it all bled good. The first pedal application works fair
but a second or third pedal push gets weaker each time. It's like the booster
doesn't recover it's vacuum quick enough. With the engine off
it looses vacuum very slowly to about 15 and holds. New boosters must
be made of gold with the prices I am seeing. Do these cars need the
reservoir in the back working for the system to have volume to work?
Any advise on this is appreciated.

I am not sure what you mean by weaker.

When you have a lack of Vacuum Issue or the Booster has an issue the Braking is harder because the Booster is not helping. When braking is harder on you then it does not brake as well.

The vacuum reservoir over the Fuel Tank is for the locking system and there is one or 2 check valves at the fire wall that are supposed to isolate that part of the system.

If you step on the brakes and the pedal sinks or you need to pump the brakes you still could be having issues with bleeding the brakes.

Thanks the pedal stays hard and does not sink. I have isolated the vacuum lines
to only the booster and the problem does not go away. My door locks do not work
so I was wondering about the trunk reservoir being involved in the complete vacuum
system. Probably not. Probably time for a new booster but I don't want to over look
something. Car had good brakes when I bought it last month but had been in storage
for awhile.

Unscrew the check valve on the Vacuum Pump and if the parts are still inside of it hose it out with WD-40.
The pic is a piston type vacuum pump but the check valve is circled on it.

PDF checking the brake booster.

The check can be done with a hand held vacuum tester.

A good used booster is an option.

Diesel911, how many MityVac strokes does it take to evacuate the booster to 20"Hg?

Sixto
83 300SD