when the same thing started happening in my jeep, I pulled the pedal
mechanism and found that the teeth that lock the e-brake pedal in place
were worn down.  At that point there wasn't much I could do about it,
it was just old, so I replaced the whole thing. I don't remember what
it set me back, but it wasn't ungodly expensive and the replacement was
pretty simple... On my Jeep, at least, there were just two bolts
holding it to the firewall.