This does not sound like a parking brake issue. It sounds more like you have a master cylinder issue.
Basically, if you have a leak in the master then you are going to have a weak pedal and it will drop when you press it until it is able to supply enough force to stop the vehicle. This can also cause the vehicle to suddenly lock the tires up, so it matches pretty closely with what you are seeing.
Here is a guide that will help with this:
If it were me, I would suggest capping off all the ports of the master cylinder and then press the brake pedal and find out if you have a hard pedal.
If not, you need to replace the master. If you do have a hard pedal, then you have a leak somewhere else in the system.
I attached the process below from the manual that will help with this so let us know what you find. Thanks
Sunday, November 14th, 2021 AT 6:52 AM