What is the reason why the dowty retarder of slow return speeds up the return journey?

Due to the fact that the dowty retarder often appears "flying top" in operation, which causes vehicle off-line, in the early 1990s, we replaced the return valve plate of dowty retarder with slow return return valve plate. The principle is that when the dowty retarder returns, due to the narrowing of the return channel of the hydraulic oil, the return journey of the hydraulic oil is blocked and the speed becomes slower, so that the upward "momentum" of the deceleration item during the return journey is weakened, and the impact of the piston rod of the dowty retarder on the shock seat and the connecting parts is reduced, so the problem of "flying top" of the dowty retarderis basically solved. But in the maintenance, the retarder with slow return is installed. After a period of use, there is still a problem of fast return. Why is that?

After decomposing the dowty retarder, we found out that the problem occurred on the piston ring. When the new piston ring was replaced, the dowty retarder returned to normal.The speed of slow return is caused by dowty retarder piston ring seal failure and hydraulic oil leakage. Due to the relative sliding between the dowty retarder piston and the retarder sliding cylinder, it is necessary to leave necessary clearance and there is oil leakage.