car brakes acting up?

Replace Master cylinder.

We have no idea what's acting up so have a good mechanic take a test drive and inspect the brakes.

Frozen parking brake cable.

If the reservoir fluid level is still good, it's probably an internal leak (weak master cylinder seal) but don't discount it may also be an external leak (wheel brake cylinder). You don't need to lose a lot of fluid to get a spongy pedal.

When seals wear, brake fluid bypasses them and you lose a little line pressure. This can make the pedal feel a little spongy and/or increase pedal travel until you pump it back up. I'm not sure whether this is linked to being out in the cold all day, or just a coincidence. A brake tech will be able to diagnose the problem, though if it's an external leak you can sometimes see brake fluid "soak" around the drums.

Not a lot of suspects. It could be the brake booster valve sticking closed (maybe ice?) or an ABS controller acting up. I rule out the master cylinder because I can't imagine a way it would refuse to budge, and if it did budge it would at least do *something*. If the brake booster valve is not operating the pedal would feel hard when it was not working and you should have a barely noticeable amount of brakes that got stronger as you pushed harder. The ABS controller is a long shot; it should show the ABS warning light if it was acting up. My money is on the booster. If it happens again, press very hard (both feet would be good) and see if there is some braking action that cares just how hard you push. If so, the booster is bad. If the booster works it will run out of effect before you push very hard.

Had the car parked underground drove to work all is well. car parked outside for the entire day on monday in the extreme called, on the drive home the brakes started acting up, id step on the brakes and get nothing until id release and get back on then right away to get pressure. checked the fluid level when i got home from work ant is was at the MAX mark. what could cause this, Vacuum leak? bad master cylinder? air in the system?

Thank you

Get it looked at.

So? Have them changed before you kill someone!