First, the accumulator contains a fabric bag filled with desiccant (a granulated substance that absorbs moisture to maintain system efficiency and prevent acid formation). The desiccant has a limited capacity to capture and trap moisture before becoming saturated and losing effectiveness.
Second, over long-term use and component vibration the desiccant granules mechanically abrade. As a consequence of saturation and abrasion, the grains gradually break down into a powder. Once the powder is ground fine enough it washes through the walls of the bag and distributes throughout the system, forming a dark gray-brown coating. Where the powder accumulates locally, it has the consistency of a fine abrasive mud. The net effect of the 'mud' is to clog orifices, "stick" small moving parts, interfere with heat transfer and hinder overall system efficiency.
Third, the accumulator functions as a filter, trapping some of the metallic and polymer particles that break free from the compressor and get sliced off of o-rings.
Unfortunately, a welded accumulator cannot be cleaned and its desiccant cannot be dried or replaced. Essentially, the A/C accumulators used on our trucks are "consumable" components that possess limited service lives. So, the rule of thumb is to replace the accumulator anytime a major component (such as the compressor, condenser, evaporator, or expansion device) is replaced. To accompany accumulator replacement, if there is evidence of 'gray mud' or aluminum/Teflon particles lodged in the orifice tube filter screen, the system should be thoroughly and methodically flushed.
IMO, A/C systems should be flushed and their accumulators replaced every 7-10 years, even if they are still functioning properly - just to address the problem of accumulator service life.
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Concerning the rear A/C on your Suburban, remove the rear expansion valve. Disconnect the A/C lines from the rear evaporator core and from their connection to the front lines near the firewall. Flush the rear evaporator core and all of the rear lines independently.
Whether the accumulator was previously replaced or not, remove the accumulator. Remove and inspect the front orifice tube. If the orifice tube is clean and free of debris, reinstall it with new o-rings. However, if the orifice tube has debris lodged in its filter screen, or if it is 'muddy,' disconnect the front A/C lines (including the lines to the compressor) and flush the condenser core, the front evaporator core, and all of their lines.
DO NOT FLUSH THE COMPRESSOR or any line while it is still connected to the compressor!Pour the appropriate amount of fresh refrigerant oil directly into a NEW ACCUMULATOR and reassemble the system using a new rear expansion valve and all new o-rings. A "universal" rule of thumb is 1 oz of oil for each condenser or evaporator flushed clean, 2 oz for a NEW accumulator and, because of the length of the lines running to the rear, an additional 2 oz for the pair of rear lines. For specific oil capacities and component replacement procedures refer to Section 1B of the
1987 Service Manual. Evacuate, leak check, charge and enjoy some cold air!
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