What I've seen, and it sounds like you may have already checked this - but are your PIN diodes pair ok?
Check C33, a 1uF 50V part - it's part of that section - this may simply be a trickle charge unbalanced state.
What does that mean?
Well the S-Meter for the RX side, is weighted by a small signal - simply to keep a set to two diodes forward biased. There is a resistor in the PIN diode section that keeps this trickle charge floating. It helps quiet the RX, but then also is 1/2 of a voltage divider - when strong RF comes in, the Balancer uses diodes to rectify the current to take it out of the circuit - in doing so, allows an amp / switch to turn on and allow more current to pass into the PIN section as an effort to attenuate the RF signal.
It's R46, a 4.7K - this is the other part of that balancer - it's there so RF "noise" doesn't do any overkill on the PIN Diodes - so they can do the job to keep the RF amp quiet but working and seeking CB band - not just everything else...But, if C33 is shorted out - R46 is taken out of the circuit, and current flows into and thru that TR12/TR48 amp section and the S-meter sees this "trickle voltage".
I clued in on this when you said you had problems with the Squelch. So I had come across this before and it was really from a re-capper I had the blessings to work on. Dried out caps cause simple problems and when you try to solve them, your off on a wild goose chance tracking this "phantom" down only to find that a cap that is supposed to stay open - didn't...
Now maybe I have the circuit description wrong, but I know the C33 very well.
Hope this helps!