Title, my winter set seem to lose pressure slowly, one day I pumped them to 38fr 42rear and after a few days they go back to 34/38, now they are at 34/34(with rear left staying at 38). They do not drop below 34 and temp swing is not that big.

Kinda odd, don't know where the problem is.

 

The tyre pressure will increase when ambient temperature is warmer and decrease when it's colder.

The pressure will also increase from a cold start due to warming of tread / tyre from driving.

Other than these variations my tyre pressure indicates the same from day to day, week to week, excepting punctures

Just thought of one further variable, if you look at pressure from tpms then sometimes the pressure isn't updated until you have driven a short distance (less than quarter mile in.my experience)

 

As Paul_59 noted, pressures will fluctuate with ambient temperature so depending on the temp swings where you live, it might be perfectly normal. Do you have a dedicated set of winter wheels you have the tires mounted on? If you live where they throw down lots of road salt, it's not uncommon for the lip on the backside of the wheel to corrode over time which can cause a slow leak around the bead of the tire.

 

Just physically check today and all 3 except rear right seem to be okay, and match TPMS number on dash.

Now I think the rear right might be the repaired one(was slightly bent before).

I will keep an eye on it. I do have a separate set of rim for winter tires, I stored them in my shop this spring so im no longer sure if that one is the repaired one.

If it gets too bad I probably have to remount the tires or get a new rim.