L322 gm box no reverse brake pedal pressure lost

Hi guys, I'm ppg from pboro,my 05 l322 has lost reverse gear,brake pedal seems to of lost 50% pressure . Sports gears shows only 1 2 3 on dash. Gear oil and filter just serviced,dexron atf 7ltrs in yesterday,still the same? Any ideas guys ? 150k on the box. Thick filings on the magnet,but never been serviced it looks? Any ideas on possible failure to reverse plzzzz

What battery voltage do you have? Not sure Dexron is the correct ATF for GM box if your car is the TD6.

It could just be a solenoid or valve body issue. I would certainly want to rule this out before moving onto more involved running gear issues e.g. reverse clutch or low & reverse clutch faults.

Dexron III (now VI) is the correct fluid for the GM 5L40-E transmission.

For the TD6 : “When the vehicle is stationary, to drive off the driver can select 1st or 2nd gear in high range, or 1st, 2nd or 3rd gear in low range. Any other gear selection will be rejected by the EAT ECU”

(For the M62 V8, only 1st gear can be selected when stationary in high range, or 1st, 2nd or 3rd in low range)

Phil