Just a bit of additional information to illustrate what has been said already.
I would suggest that the SMC piston is not able to return under pressure of the spring due to contamination / rust / crud at the top end of the SMC, or due to the cylinder bore becoming rounded.
When the brake pedal is pressed, some of the pressure goes direct to the rear centre piston. Another line goes to the SMC. The fluid enters the inlet port of the SMC which is in between the primary and the secondary seal. When the bike is in the garage and not moving, the pressure from the fluid flows past the outside of the primary seal and applies the rear brake by extending the two outer pistons in the calliper. This is normal.
However, if the SMC piston has not returned properly under spring pressure, then when the rear brake pedal is pressed, the pressure going through the SMC will be able to provide a little extra push against the secondary seal (the one closest to the rubber boot), and push it back - ie in the direction of the rubber boot. This may happen if the extra pressure required to push the SMC back is less than the pressure required to apply the brakes. But by this time, the blockage in the SMC is already enough to prevent the SMC piston returning under pressure of a fairly strong spring.
The symptoms of this are that on the first press of the brake pedal, the braking feels a little soft, and there is too much travel on the pedal. After that, the brake pedal feels normal again. However, each time the SMC moves, the SMC plunger is still not returning and the rear brake will be dragging. The rear pedal will feel soft again when it is able to push the SMC back.
Applying the rear brake pedal also activates the centre pistons of the front callipers, which in turn will cause the SMC to activate. So you cannot prevent the SMC from operating whichever you use - the pedal or the lever.
However, your quick dab of the pedal is probably allowing the SMC piston to be pushed back to its normal position. It doesn't have to be a quick dab - it can be a gentle press - as it has nothing to do with the delay valve. This state of affairs may last for a short while before the extra pressure from the brake pedal returns the SMC to its proper position. The first use of either brakes to slow the bike down, however, will put things back as they were with the SMC not being returned.
The symptoms are a sure sign that there is something wrong with the SMC and it is about to fail completely - resulting permanent brake drag, very hot callipers and brake disc and rear wheel locking. While it is true that you can get home by gently depressing the brake pedal after each use of the brakes, this situation mustn't be allowed to remain unaddressed.
These observations are taken from a friend who had this problem. He reported that the rear brake pedal would go 'long' after riding about 20 miles. The first press of the pedal cleared things up - he heard a clunk as the SMC returned to its proper position - and the brakes worked ok after that and the cycle would repeat. He did not complain of rear brake drag however, and he was not losing fluid. My suspicions in this case were as described above, but that the SMC cylinder bore was so oval that the primary seal was failing and fluid from the rear calliper could squeeze past it to relieve the pressure in the rear calliper.
I dissected his SMC after he had replaced the whole SMC assembly -
the photos are here - Post #1. This is little over one year after fitting a new service kit !