Got a weird sporadic problem with my brakes

Seems to happen more when it's really cold (-30ish). Basically start the truck, drive about 2 blocks. Hit the brakes the pedal suddenly goes hard and brakes don't grab. Wait about 20 seconds or so and the brakes return, pedal is normal, and everything is fine. What the heck??

Truck is a 91 4runner SR5 with ABS. This is the second 4Runner I've had this the same issue. They both did the exact same thing and they both had ABS. First truck was a 90, this truck is a 91. The problem only occurs first thing in the morning after it's been sitting all night. After it happens it's fine for the rest of the day even after it's been sitting for 8 hours at work

Any ideas?

 

Sounds like it could be booster related.

Confirm that the check valve is working properly and confirm that there are no vacuum leaks.

Check your booster:

1. With the engine off pump the pedal until its completely hard. Start the engine and note two things: (as already mentioned above) did the pedal soften at all? Second, after a say 10 seconds of idle does the pedal feel any better.

2. The second thing to check is if vacuum is getting to the booster. There is a plastic check valve on the vacuum line going from the manifold to the booster side. Carefully wiggle/twist the check valve to momentarily break the seal on the hose on the booster side. If you hear a hissing, you are getting vacuum to the booster. If not, check the engine side (probably not a booster problem). Go from there.

If #1 is NO and #2 is YES on the booster side, you have pretty much confirmed your booster is FUBAR.

Good luck.

 

Swapped out the power brake booster and looks like it did the trick
Thanks for the info!