Need under dash ac help

[norm]

I have a 55 Belair that has an under dash unit. When I start the car the gages low and high read what they should for the ambient temperature of 80 deg. 175 /45. After running for about 20 minutes the copressor starts to short cycle about every 30 seconds or so. The gages then read low side 30 high side 175 this is when the copressor starts back up. The evaporator unit is set at high fan, and the thermostat is at max cool. The unit is pulling down to 47 - 50 degrees. Any ideas on what to look for ? ps 134a unit.
Thanks Norm

[mech393]

if you have a site glass on or near your receiver dyer when its running see if there's air bubbles flowing through it. It seems it could be low on Freon or possible there's air in the system . if push comes to shove i would evacuate the system and pull a vacuum see if there's any leaks, remove the moisture, replace the dyer and install freon back into it , temp should be around 38 degrees out the vent .