It will leak pressure Into the engine but should pressurize it enough to heat and major leaksI made one of these caps last year and even installed a pressure gauge on it. Couldn't get the system to hold air pressure and would leak down my compressor tank in no time. Couldn't find air leaks anywhere but you can hear air rushing through the sytem. Not sure what to think.
Last time I had it at dealer about 3mo ago and complaining yet again about the regens they said they tested the EGR and no issue. Also said they checked for boost leak and found none but tightened clamps anyways. No change in regens.the egr valve could be not sealing and letting it bypass the engine is my thought. and would explain your short regen cycles.