Background - Both CP3 and CP4 are experiencing obstructed "high side" sensing lines which prevent their level transducers from measuring the level of LN2 in the 80K pumps. We consider that this is due to accumulated ice at the bottom of the pumps inner LN2 vessel where the "high side" sensing line transitions from the relatively large ID aluminum tube to the relatively small ID stainless tube near a 90 degree elbow. While Chandra R. has developed a procedure for a permanent post-O2 fix, we, nevertheless, would like to pursue more accommodating fix attempts in the meantime. One of these is to see if by applying a vacuum (for a prolonged period) to the sensing line enough ice could be liberated through reverse sublimation that the obstruction could then be freed by alternating applications of modestly pressurized UHP N2 followed by vacuum pumping. Today - WP #6442 Kyle R., Chandra R., Dave B., Gerardo M., Marc P. I reconnected the "clogged" sensing line (with added parallel valved line used for vacuum pumping and a parallel valved line used for back filling) to the CDS level transducer (Rosemont model 1151DP4E52B404). I then applied various "on-scale" pressure differentials via the back fill line (as indicated by the parallel connected magnahelic DP gauge) while Chandra R., Dave B. and Gerardo M. monitored the CDS MEDM screen. This confirmed that the transducer was operating and was in rough agreement with the mechanical gauge. I also switched to the application of vacuum and they noted that the 4-20ma transducer output fell to 0 mA or something less than 4 mA (thought 4 mA output was minimum output?). I then rearranged the plumbing fittings so as to permit selecting between a vacuum pump or a back fill line (10 psi UHP N2 @ 2 LPM) without the need to undo any tubing fittings as had been the case initially. During this plumbing rearrangement, the sensing line was open to the room and began to flow GN2 past the obstruction. This phenomenon had been observed on previous dislodge attempts (see entries from last month?) the difference this time was the significant increase in volume and duration - "Almost got it!" I experimented with various durations of pumping and back filling etc. and observed on a few occasions that the audible gas load to the diaphragm pump would change spontaneously if let alone for several minutes. Anyway, this is all very encouraging and will likely be continued next week. I halted tonight's attempts in observance of the Vacuum Team's Friday Rule and to allow others (the larger vacuum community) to consider if this approach is thermally cycling the bi-braze joint and poses a risk. I am leaving the sensing line back filled to 10 psi and isolated from both the vacuum line and back fill line.