I have had a hard brake pedal on an '83 240D on and off since a brake master cylinder replacement several months ago. Initially, it seemed the cause was a bad check valve on the booster hose. I replaced the entire hose and valve with a new one. Tests with a gauge and a hand pump show that the vacuum pump is producing good vacuum, the check valve and hose are holding pressure, and the booster itself is holding pressure.

I tested the booster itself by plugging some fuel hose into the spot where the main hose connects. It was just the right size to fit snugly after some pushing and twisting. I used a hand pump to draw air out. It took a while, but after 45 seconds or so of pumping, pressure started registering and holding on the gauge (this is similar to what happens when you do a direct test on the vacuum reservoir).

So my question is: what else can cause a hard brake pedal? Can the booster fail mechanically; that is, can there be another problem with it besides the inability to hold vacuum?

I should also note that I have some vacuum assist on the brakes, but the level varies, and it is never full assist. I can't determine a pattern to the level changes. There is a loose correlation between spending time on the highway, and lower levels of vacuum assist, vs. local driving and more assist, but I'm not confident this isn't coincidence.

Just a guess. Maybe you should re-bleed all the brakes starting with the rear. May be some air in the lines still. Does the MC still have fluid in both front and back compartments i.e. are you sure both are filled?

How would you explain how air in the lines could possibly result in a hard brake pedal?

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

One common cause of intermittent vacuum problems is a failed check valve at the vacuum pump inlet. The valve parts rattle around in the pump inlet and intermittently block air flow.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Can't now that I think of it. But never a bad idea to re-bleed brakes.

I have experienced a hard brake in the past but do not remember what caused it. Was not the MC because I still have the original.

Bad brake booster usually results in extreme extra effort pushing on the pedal to stop the vehicle. Air in the system is typically a mooshy pedal. Do not quite remember what causes hard pedal but someone should know; it is not a new problem.

Just read your post above and the idea of loose parts in the vac pump inlet does make sense.

It would be better if you posted exactly how much vacuum you are getting at the Power Brake Booster.

Also there is an O-ring between the Master Cylinder and the Booster.

If for some reason the rear seal on the Master Cylinder leaks fluid goes into the Power Brake Booster (have you been having to add extra fluid or is your fluid low).

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84 300D, 82 Volvo 244Gl Diesel

I'm betting the fibre check valve in one of the primary hoses to the vacuum pump has failed, BT, DT more than once on higher mileage cars .

Just undo the hose where it attaches to the vacuum pump and them unscrew the check valve ~ you'll see the disc is good or has cracks/chunks gone missing / whatever, blow through it too, one blows one way, natch .

-If- you find the check valve is bad, run the engine with the valve removed & port open at raised RPM's until the little bits of old fibre valve disc come flying out.....

My brother's W126 had this and he screwed around for several months with the randomly hard brake pedal before allowing me to undo the primary hose/pipe and blowing the bits out, never any problem since then, over ten years now .

Give it a look anyway .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

This is an odd issue, a kinked, clogged or swollen brake hoses are possible but very unlikely.
I'm siding with Nate.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Typically if you have any obstruction the brakes will brag or the cluth will be very slow to take up .

This is far more common than most realize but not the primary issue here I don't think .

I just finished a brake job that had almost 1/4" of crud inside the rear brake cylinders, finally it got so much crud built up some chipped off and damaged the rubber cups, causing a serious brake fluid leak, why it was brought to me for repairs, not because the brakes were poor (they were) .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Tried to have a look at this today, but couldn't get the allen bolt on the hard mount (by the fuel filter mount) loose, hence couldn't move the vacuum line out of the way of the check valve on the pump. I didn't have much time, so I had to give up; I'll try again in a few days.

Interestingly, though, the pedal was close to normal for most of an afternoon of driving after my failed attempt. All I wound up doing was loosening the main fitting at the pump, and tightening it back again when I couldn't get the allen bolt loose.