I can't get my water temp. over 77 deg. I checked the volt on my 2 heating elements and they read 120v. However I get no voltage reading on my pressure switch, should I ?

did you check voltage ACROSS the heating element terminals or did you go from each terminal to ground or neutral? Always check across the terminals, depending on your spa type, you will either have 240V (most common) or 120V (if it's a portable spa). Your pressure switch could very well be a low voltage type and may be between 12 and 24V DC depending on model. You won't read any reading on your AC scale on your meter. Check across the switch terminals (with wires disconnected) on your OHMs scale and you should have no resistance there.