My RPM's on my Prochem 405 don't seem to be what they once were.

Does this loss of power occur during load or no-load? If this occurs during load then your gov. oil may need to be changed.

Read the manual one more time Johnny. Where does it say "diaphram?" The manual says "Vacuum relief valve requires adjustment or has a vacuum leak due to damaged gasket".
Visually and physically inspect the gasket to see if it is separated from the lid at any point around the edges. And if your vacuum gauge has dirty glycerin like you said, get a new one or change the glycerin. And never use the gauge off the machine if you think there is a problem because those plastic lines are unreliable.

Also make sure the belt is not slipping when is under load. you can check it if you mark the belt and the pulley with a marker or something at the same spot, then turn on the machine, then block the vacuum port, then turn off the machine, then check if the marked area match, if not the belt is slipping.

You need to check the joint where suction tube connects machine console to waste tank. This can be a rough connection and debris can lodge there causing lint to further clog it up. Tooth pics, hairpins small sticks and pieces of mulch sucked up when throwing down vac hose outside home after jobs are the culprit! If clogged, try 1.5 in hose on shop-vac inserted as far into front of vac inlet to reverse suction and clear debris. You may also use drain snake to help dislodge then suction in reverse. My Legend has the same joint and has been clogged so I am always mindful of sucking mulch by accident.

Answer my question Johnny. What is your current Hg lift reading in high speed with vacuum inlet on front of machine blocked off?

STOP!!!!!!!!!!!!!! It should be 12/13 under full load and on high. That is with both front vac ports completely sealed off. 30'' may implode your dump tank if it is not hurt already. 10'' with no hoses sounds like a blockage, a bad one. Run a solution line through the front ports to knock anything loose. Check the filters in the tank. Clean if necessary.

I bought my Performer 405 used. I found this in the elbow. 4" diameter elbow choked down to 1".

Your govenor may be bad. Check with Keith of A@A supply of Texas. Or local towmotor shop

That joint on a Prochem is a clog waiting to happen. When it clogs, your HG gauge jumps, wand suction drops and when your dump valve opens it makes a gurgling noise. I'd like to do away with the whole "through the console" vac tube and add 2.5 in connection straight to the tank. None of the new PC TMs have such an over engineered arrange for connection to waste tank.

You need to hook up a tach and see if your even at the correct rpm for your setting.

From front of console, vac tube bends 45deg left then right to back of console. Rubber 2.5in wire core gas pipe connects to tank inlet. If clogged, it will most likely be at the rubber pipe connection. I first detected my clog with drain snake fed into vac inlet as far as it would go, pulled it out and there was lint stuck on the end of it. It took a few rammings of the clog before it came loose enough to suction out. You can also ram clog from other direction but there is a nasty 90deg bend inside tank.

I'm done with you Johnny. You don't listen to people. Your gonna ruin that machine. I've never dealt with anybody so ignorant and dense as you. I hope you don't hurt anybody with your truckmount. Happy trails!

Maybe instead of typing words, you should just post pictures....and just one at a time.........per day. Just sayin'.