I don’t fly the SP but I’ve got around 3,000 hours on the 109S. As already stated, parking brake “unreliability” does vary from airframe to airframe on the 109 series. It's one of this aircraft’s dirty tricks, so it’s always best to chock a main wheel. If you come back to the aircraft and the wheel is leaning on the chock, don’t pull the chock out! Silly system, tricky if you’re single pilot with no ground crew to remove and stow the chock once you’ve started the aircraft and pumped up the accumulator. If possible, borrow a chock that can be left behind and park nose up slope, so you can taxi forwards.....or come back for your own forgotten chock later!
BTW, for similar reasons, NEVER leave the rotor brake lever in the ON position. Stop the rotors then move the lever to the off position. If you don’t, one day you might start the aircraft with the brake selected on and because the pressure will have bled away, it will provide no resistance until the hydraulics rapidly pump up...... and then it gets very smokey and very expensive very quickly. I know of two unfortunate 109 pilots who did this. It cost one of them his job and it was his first day! It’s easily done if you’re used to aircraft with a proper rotor brake system. Why the manufacturers never put an interlock on the system to prevent the engine starting with the rotor brake lever on is difficult to fathom (unless they saw common sense and there is one on the SP, of course I’m talking slightly beyond my knowledge base). Even the military Gazelle had a stout piece of wire across the throttle lever to prevent it being advanced with the rotor brake lever in the on position!