a/c compressor cycling too fast
 

i have a 77 g/prix, when i got it the a/c comp. would cycle constanly. so i figured it was low on freon, converted to r134a does the same thing. if i bypass the temp switch [ the one with the probe]compressor runs constantly so i figured switch was bad ,put a new switch on does the same thing, what else controls a/c clutch. it does blow cold when switch is bypassed

Low pressure switch is different for 134a, some of them are adjustable. Low side switch should open around 22-23 psi. You need to put gauges on the system and see what's going on. Post your idle pressures, high and low with the switch jumped out.

Found the issue, someone put temp switch probe on the a/c line that gets real cold quickly. I moved to the evap core seems ok now

Glad you found it. 1977 Firebirds used an evaporator temp switch to control clutch cycling, so I didn't know if the GP did too. By 1979 for sure I've seen pressure cycling setups only.

Pressure cycling switch on the accumulator became the standard for decades after that. I love the simple orifice tube systems.