Type 1 Gas Lock (World Oil June 1992 Vol. 213 No 6) occurs only if the fluid level is greater than 1/3 the total depth to the seating nipple or if a bridge is present that gives the effect of pumping from below a packer (rare). Expanding gas bubbles "tickle" the balls and seats (valves), and the well comes to equilibrium pumping at very low efficiency and with an elevated fluid level. The cure for Type 1 gas lock is to use either a snubber cage or a backpressure valve. I prefer the backpressure valve because it is easier to adjust and repair.
Type 2 Gas Lock (World Oil June 1992 Vol. 213 No 6) occurs when a volume of gas is trapped between the valves in a pump. In a Type 2 gas lock, the peak pressure of the trapped gas on the downstroke is insufficient to overcome the hydrostatic head on the traveling valve. Then, the pressure is not reduced enough on the upstroke to allow the standing valve to open and admit new fluid. Both valves are effectively stuck in the closed position and the pump refuses to pump. This is essentially the opposite of the Type I gas lock, but the results appear the same. Using a backpressure valve is not the cure for Type 2 gas locks; the practice will actually make the problem worse.
(Paraphrased with the Authors Permission)
The picture at the left shows a bottom hold down insert pump in the fully closed position as it would normally be sent out from most shops before 1992. It has a conventional seat plug and traveling valve assembly. The red area is that part of the pump where a TYPE 2 gas lock would occur. The best strategy would be to decrease the volume in the "red" area. That would give a higher compression ratio to the pump and the pressure on the gas could conceivably raised to a point where it would overcome hydrostatic load on the traveling valve. (Compression of this pump would be about 28:1 and it would gas lock easily at depths below 3000 feet and stroke lengths less than 120 inches) As pumps are run ever deeper the hydrostatic load becomes greater and the value of increased compression rises. Common methods of increasing compression include (1) Longer valve rods, i.e. cut the valve rod such that TV to SV distance is 1/2" or less. (2) Install "hex type seat plugs". These plugs add a small amount of length to the valve rod. (3) Use "high compression" standing valve cages. Standing valve cages designed for maximum compression are available from such suppliers as HIVAC.
Of course the most common method of increasing compression is lowering the rods until the pump "bumps". If you incorporate the mechanical advantages outlined above you will have better success even when you do have to "bump" the pump.
As an aside it is much faster to adjust rod spacing on wells that have a "solid" spray metal polished rod than it is with the more common "liner" on a polished rod type.