Are compression test gauges supposed to stay at the peak pressure during testing? This rental gauge raises and falls.

Maybe I'm doing something wrong so here's more detail:
Local shop wanted $150 for a test so I rented a gauge from the local auto store and looked up how to do it myself.

I pulled the in-car fuse that said fuel pump.
I started the car and ran until it died.
I pulled the fuse from the under-hood box that my service manual said was the ignition fuse.
Removed the plugs and put the tester in cylinder 1
Had a buddy get in and press gas and clutch and crank it

As it cranked the gauge would rise then immediately fall. This made it basically impossible to see the numbers with my eyes. I used my cell phone's video camera to record the gauge as the car cranked then watched the video in slow mo. I saw it reaches 125 psi consistently with every turn-over. I even checked on all cylinders with this method and they were all 125 but that seems low and the I have a suspicion that this rental gauge is busted.

 

Gauge must be faulty! A compression gauge will keep the pressure until you release it by pressing the bleed valve. I have a cheap one and it works just fine, so I'm pretty sure the gauge you've rented is utter crap. According to Bentley, compression for a base cooper should be between 167-247psi and a Cooper S between 131-196psi (with a max difference of 7 psi between cylinders).

 

One thing you can feel good about is that all four cylinders are doing the same thing, as they should.

 

Got a new gauge and it holds pressure. Firdt cylindar tested to 155 psi.

However the tester is now stuck in the spark plug hole and theres no way to use a wrench to turn it. Im letting the engine cool while I pray, very hard.

 

Finally.

Here are my numbers for future searchers:

#1 155
#2 145
#3 144
#4 147

 

Apparently your prayers were answered!