Weird Brake Problem

[Groover]

The brake pedal goes hard the first time I go into reverse in the morning. 3.5L EB

1. Press the brake pedal
2. Start the engine
-> brake pedal softens over two to three seconds as vacuum increases
3. Wait a few more seconds
4. Go into R
5. Release brake pedal
-> vehicle starts moving backwards
6. Press brake pedal
-> pedal has gone hard, hear a groaning noise as I press it
7. Release pedal
8. Press pedal
-> works normally

I can then drive it for 120 miles (for example) without an issue in D, R, etc. The next morning it happens again.

I can induce the problem by turning the engine off then pumping the pedal to remove the vacuum then going through the steps above.

I don't want to just throw parts and money at the problem, I would like to diagnose it. However as the check valve is cheap I am going to put one of those on there.

The FSM states that if the pedal softens after engine start then the brake booster is working fine, so I guess that isn't the issue. The FSM doesn't really discuss diagnostic steps for the vacuum pump.

I don't have any warning lights on the dash so I presume that means the vac sensor is working and vac is in the correct range.

Any ideas or suggestions?

Thanks!

 

[Groover]

On Saturday it didn't do it at all and then it was back on Sunday. :/