04 tundra transmission

My 04 tundra with close to 370000 miles is starting to have its first transmission issue. When I break heavily the transmission slips out of gear into neutral. If a wait a few seconds or give it little gas it goes back to first gear. Has anyone else had this problem? Any suggestions as what could be issue?

Check your transmission fluid level. It sounds like the fluid is moving away from the pump inlet under hard braking. In any event, if the fluid and filter have not been changed in the last 60,000 miles, have that done right away

It has never had any service. From what I understand it’s a sealed unit. At one point I debated having the transmission fluid replaced because of the high mileage. I was told at this point it may cause more damage to open it up and mess with it and changed the fluid.

Now it’s having this slipping issue and I don’t want it to get worse

It will not cause damage. It’s not sealed, get it serviced

Thanks I’ll look for repair shop

You may want to find an independent transmission shop here: https://members.atra.com

At 370,000 miles I think your transmission has served you well.

Agreed. It’s been a wonderful truck. Original engine and original transmission. The only thing I’ve done to it is routine maintenance. A few timing belts, brakes , suspension, radiator, u joints, ball joints lots of oil changes. I’d love to keep her going at least another hundred thousand miles. I’m the original owner.