So, if your boss comes in one day, and asks "who wants to learn how to fix a vacuum pump?" say no. It's a trap. Before you know it, you'll be neck-deep in fetid, black pump oil as pump after pump after pump breaks down.

Here's how it might go:

After a while, you'll find that your vacuum line is starting to lose some of its original pull.

First, you should regrease everything.

Then, you should change your pump oil.

Or perhaps your pump is leaking oil.

Now you'll have to change some of the seals. Bummer!

It took me a while to find any sort of instructions about seal replacement, so I'm going to pass that onto you, the fine denizens of the internet. I hope this helps someone.

Step 1: The Precision Pump

These instructions are for precision pumps - I know that they're the same for the D25, D75, and D150. Your mileage may vary.

Every time any maintenance is performed on one of these pumps, there's a seal around the shaft that needs to be replaced. The most common cause of leaking oil is this seal. If it isn't replaced when the pump is fixed, you'll end up with a steadily growing puddle of oil, and a pump that pulls a poor vacuum and is prone to overheating.

Step 2: Tools and Parts

Get yourself the following:

An alan wrench set
A flathead screwdriver
A leatherman or other set of pliers
A hammer (yes, seriously)

A precision shaft seal and cover gasket

I buy the parts for my lab's pumps from Capitol Vacuum.

Gloves, especially if this is a laboratory pump with god-knows-what in the oil, are a necessity.

Step 3: Drain the Oil

Don't forget this step, or you will have a hell of a mess on your hands. I forgot once. It's the sort of thing you only forget once.

Position something to collect the oil underneath the oil spout. If the oil has been recently changed and still looks golden yellow, it can be reused.

Step 4: Disassemble!

Before you can get to the innards of the pump, you need to remove the gas ballast valve and wheel.

The gas ballast post can be removed by unscrewing it with the leatherman. It should lift right out.

The wheel is held onto the shaft with a small hex nut. Take the belt off, and remove the nut with the corresponding alan wrench. This is where you might need the hammer - sometimes the wheel needs a little... "persuasion" to come off.

If the belt is being difficult, there are four extremely hard to reach bolts underneath the motor that, when loosened, allow it to slide back and forth.

Now you can crack open the innards of the pump. Remove the hex screws on the faceplate of the pump in an alternating manner, as if you were changing a tire. Carefully, pull the whole assembly straight out and set it aside. You should see part of the shaft seal stuck to the shaft.

It's surprising how rusty the innards of a perfectly functional pump can be. Don't be worried, they can take a lot of abuse.

Step 5: Replace the Seals

Remove the old shaft seal, including the piece that remains stuck to the pump case. You may need to use the screwdriver to poke it out, but be careful not to gouge the housing.

Replace the shaft seal. The part that fits into the housing may need a fair amount of pressure to wedge in, but resist the temptation to use a tool - scratching the surface can lead to oil leaks.

Remove the old gasket, and replace.

Step 6: Reassemble the Pump

Carefully re-insert the pump assembly into the housing, taking care not to damage anything. Thread the exterior screws through the gasket, and tighten them in the opposite order they were loosened in.

Reinsert the gas ballast valve, put the wheel and belt back on, and you are ready to go.