I will give you one of the reasons (which is the most common) why your vacuum pump may not be working correctly or not have the amount of ‘vacuum’ it used too.
I will be referring to the one pictured – Diamond Tool Industries Vacuum Pump
These vacuum pumps are a very simple unit and generally have little to no problems with them. As long as you follow the maintenance required, which is as simple as squirting a few drops of oil (compressor oil or similar) into the suction barb and run for a couple of minutes before switching off and packing up for the day, then you will have no problems.
So if next time you go to a job and set up the pump, turn it on and then feels like there is no vacuum, the above maintenance most likely was not done! What generally causes this is there has been water or moisture get into the pump and while sitting over time has created these little fins or veins to get stuck onto the main shaft wheel.
Pump will still run, however there will be little to no vacuum.
The good news is this is easily fixed, providing it is the problem with the veins being stuck in place.
Simply unscrew the allen key bolts on the front of the pump (where the 2 barbs are) and take the front cover off. Be careful as there is a spring and basket type filter.
Unscrew the next plate which has the barbs on top and remove.
Now you will see a round cylindrical wheel with 4 veins in them. The veins are small rectangular sized. They slot into their allocated grooves and move in and out when the pump is spinning to create the vacuum effect. Just make sure the shaft spins freely by gently turning by hand.
Take the veins out carefully, pry them gently if you have to but be careful not to damage or break them. Give them a quick wipe over and lubricate with some oil and put back into position. They should move freely in and out of the groove. Do this for all 4 then replace the spring and basket filter and screw front cover back on.
Start your vacuum pump and notice if there is more vacuum. If not a further diagnosis might be required or it might need to be taken to a repair store.