So the brake pedal on my 83 300td seems harder than it should be. I'm comparing to a 300d, and other cars as well. Whenever I switch over to driving one of those, I always end up "over" pushing the brake pedal for the first few minutes.

There are some small vacuum leaks in this car (in the shutoff line?), however when I disconnected everything else but the booster the pedal still felt the same. I didn't look at the big vacuum line much. Hooking a mityvac to one of the two small hoses that pop out of it (and blocking the other, so only the booster's hooked up) shows that the booster holds vacuum without any leaking out. So I'm not sure what's wrong with it. Ideas?

How much vacuum do you have?
I had a 77 300d with a hard pedal. It turns out the front pads were hitting the metal springs holding the pads in place so they were ineffective.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

20-24 (it was different on two different days). It goes down to 15 when the rest of the system is hooked up (that was at a different testing point though).

The car seems to stop ok though, apparently better than the guy who rearended me.

How long has it been this way ?
Did it develop gradually or pretty sudden ?
Have you done any work to your brakes lately ?
When did you last flush your brake fluid ?
Do your engine shut off and door locks work fast ?

I've got the same issue I have yet to take the time to track down.

One place to check is whether or not your booster is holding vacuum. If it isn't, aka leaking, you've found the culprit.

But as others have mentioned, it may be dragging pads, stuck calipers, swollen or clogged brake lines, etc. But an easy check is the booster's vacuum.

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I've only had the car for a few weeks. The fluid is darkish brown so it looks like it needs changing.

Like I said, there's a small vacuum leak somewhere, even with the leaky part disconnected, the pedal still feels the same (didn't really drive like that though...). The engine shuts off and the locks work, but both seem slower than normal.

The transmission is also shifting hard, but I assumed that was because of the leak.

How long has it been this way ?

Did it develop gradually or pretty sudden ?

Has it been that way ever since you got the car ?

Yes, it's been like this the whole time I had it. No idea how long before that. The car was sitting for a few months.

When I was new to MB and my vacuum booster went out... I knew I could drive it home.... so I headed home very slowly.... and the brake was so hard to push I still almost went through a stop sign on the way home... So vacuum is really the first thing to check on these old cars...
Now, if you had replaced the master cylinder... and perhaps the former owner did... and you did not adjust the free play of the brake pedal you can get this same type feeling... and perhaps be wearing out your brake pads too...
Do any of the brake parts look new ?

If the brake pedal is hard and you say you have plenty of vacume , it sounds like the booster seal is eather leaking in the back of the booster or booster is bad, now for your transmission, sounds like you have a vacume line disconected eather at the valve on the pump or at the transmission the way this trans shifts is the more throttle the less vacume you have at the trans and it is suposed to be this way to much vacume will cause it to shift hard and no vacume will also cause weard shifting. the injection valve on top of the injection pump controls this controled valcume and vacume modulator on the transmission has and adjustment also ,Jerry

Interesting that someone is having the same / a similar problem as me.

My problem started last year, around October. The vacuum would go out intermittantly, I'd lose door locks, vacuum vents, rough shifting, shutoff, and no vac-brakes. I had originally thought it was the pump, so I went out and bought a Mityvac to do some testing.

Well, thankfully I was wrong. The pump put out a constant and strong 24hg, even while the rest of the car was having vacuum issues. I've replaced the check valve in the vacuum pump, the brake booster line, and the problem seemed to go away, at least until recently. Seems the cold weather doesn't like my 300D, since as soon as it got freezing out, I lost vacuum again. Obviously, I have a leak somewhere (or a faulty check valve), but the question is where.

Any ideas where I should start looking? I just ordered a new yellow check valve for the A/C system. This issue is driving me nuts, since it's causing me to constantly pop the hood and "fix" the vacuum issue. Not to mention, pretty darn unsafe when driving 30 mins a day on a road where people regularly go 80mph.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]