If both rear brakes are locking up it just about has to be a master cylinder. Find the line between the rear section of the master cylinder and the ASR hydraulic unit and loosen the line nut at either the master cylinder itself or the ASR hydraulic unit fitting marked "V". If the rear wheels free up, focus your attention on the master cylinder and/or the seals between the fluid reservoir and the master cylinder.
There is a synthetic material relief valve in most MB master cylinder pistons of this vintage and it is extremely sensitive to even trace amounts of almost any kind of solvent. Additionally, the piston seal rubbers themselves can disintegrate and plug passages. Anybody get a nice thorough cleaning of the brake fluid reservoir using anything other than fresh brake fluid and compressed air (which incidentally, will never make it look actually clean inside)? If so there's your likely culprit. Also look for any swelling of the seals between the master cylinder and the reservoir against any possibility of them swelling and closing off the small bores that reside just beneath them. Have two new ones handy, because after you remove the reservoir, you have to replace them anyway. Additionally note that if you see or feel what appears to be swelling or softening rubber, you absolutely must replace your master cylinder regardless of any other consideration. If the fluid cannot return to the reservoir upon release of the brake pedal, then locking of the brakes could be the result. That is an extremely rare occurrence, but I have seen it.
Do not use anything other then clean fresh brake fluid to lubricate anything for reassembly.
If one rear brake is locking up that could be its connected flex hose, seizure of that particular caliper, or the ASR unit. If you find one rear brake is locked up: Open the bleed screw. if it frees up, it's either the hose or the ASR unit. If it doesn't free up, it's a seized caliper. If you have to remove the caliper, replace the hose anyway.
If it did free up upon opening of the bleed nipple, close the bleed nipple and pump the brake pedal until that particular wheel is locked up again. Find the brake line attachment points at the ASR hydraulic unit. The letter V is "Vorn" for rear, The letters L and R will apply to left and right just as in English. So, the attachment point marked VR is rear right, and the attachment point marked VL is rear left. So, get the particular rear brake locked up again, and then loosen the line for the appropriate rear wheel. If it immediately frees up, then it is either a blockage in your ASR hydraulic unit, or some other ASR related malfunction.
