As title says, I have a VERY hard brake pedal... The car does stop but the pedal is hard. I have yet to remove the pump to see if its original. I was thinking of replacing it anyway as I've read horror stories and it does have 310,110K miles on the clock and as I dig into the car, EVERYTHING is original. I would think my best bet is to hook my mityvac up, but where? How much vacuum should these pumps put out? Where do I look? Help here fellas.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

Hi Jake - I've attached a tech article on the vacuum pump replacement below. I hope it's useful in troubleshooting and addressing the issue. Best of luck!

Vacuum Pump Replacement


-Dmitry

Most diesel vacuum pumps should produce 15-22 inches of vacuum. The test for vacuum should be taken off the main vacuum line coming from the vacuum pump to the brake Booster. If you have good vacuum, then the likely defective part is the brake booster or you have a vacuum leak somewhere. With the number of miles on the car, a new vacuum pump is a good idea anyway.

The vacuum check valve on the output of the pump fails also. When mine failed same thing but I found changing the whole pump with a new check lave in my favour since the cheek valve alone is speedy.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

Okay folks I will check vacuum on the pump and check for some vacuum leaks or cracked/disconnected lines. I've found many of those recently. I think it's the vacuum pump since I'm having central locking issues as well. I can only lock it sometimes from the passenger side. But won't say for sure until I do more tests. It will get replaced regardless, along with every vacuum line in the engine bay. If it's the booster, or check valve I'll just grab those from a parts car.

__________________
Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k

My W124 2.5D vacuum pump let go at 70 MPH. The engine had 264K miles on it . It caused such a mess the engine needed to be replaced with one from a donor car. After a while, I would wake up at night wondering how the vacuum pump was in THAT engine. I replaced it and found it could have run for a lot more miles. But, I sleep better.

Recommendation: do not install a rebuilt vacuum pump. Use a new genuine Mercedes pump only. Rebuilt pumps are prone to caststrophic failure.