New here in Oklahoma I have a 94 corvette bought is used it was at a shop when I bought it. Shop put new pads and hoses and left front caliper about 10 minutes into my drive home the left front brake locked up opened the bleeder relieved the pressure and made it home. Now the strange part is you can start it and in give or take 10 minutes the brake pedal is rock hard and left brake is locked up bleed caliper pedal returns to normal. I don’t think it’s a booster issue maybe master cylinder or ABS any thoughts Thanks in advance 

The puck in the calipers is seized in the housing. Not returning when breaks are applied. Replace the caliper and rebleed.

Thanks I’ll try that still doesn’t explain why the brake pedal gets hard while the engine is running it’s fine with the engine off. When you first start it the pedal is fine booster seems to be ok after 10 minutes or so not driving just sitting at an idle the pedal is hard car won’t move. It’s like something is pressuring the system up

Sounds like the brake booster is acting up. What year is the car, how many miles on it. Any records at all on brake work?

It’s a 94 only record I have is the left front caliper and both front brake hoses were replaced 130,000 miles I bought a new master cylinder but haven’t installed it yet. I didn’t know if the ABS could be a problem but could be a booster 

The "new" replacement caliper could be defective. If the piston(s) are not retracting when the pedal is released, and the brake locks down, you basically have no room for fluid to move, therefore the pedal gets hard. WHY was the caliper replaced anyway? There is a SLIM chance there is an issue with the ABS module, considering you said you can open the bleeder on that caliper and it will release. That typically is caused by a blocked return hole in the master cylinder, which would normally cause both of the calipers on an axle to have an issue, but with ABS, things change. I might want to take a look at the ABS system, possibly have it bled properly. 

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I can’t tell you what the original problem was the owner took it to the shop to have the brakes fixed the shop replaced left front caliper and both front rubber lines. It was at there shop for sale the tags have been out since 2019 I’m guessing they just drove it around there shop for the test drive. I got it cheap they charged the owner $1100 for a O’Rillys rebuild caliper and 2 brake hoses I didn’t feel comfortable taking it back to them