Sinking brake pedel on 2011 CrewMax 2 WD

Hi, new owner of a 2011 Tundra Crew Max. Nice one owner truck, but I have something that does not seems quite right with the braking. When coming to a stop, the brake petel feels solid but if I hold it at the stop, it will slowly go to the floor. If I pump it once or twice it will usually come back up and be solid. I know its not the brake booster as I have had a Toyota 4 Runner with a bad booster and the pressure needed to stop was very hard. This takes a normal amount of pressure to stop, but one stopped at a light, it will slowly go to the floor and needs to have more pressure applied to keep from moving. If the pedal is pumped once or twice it will usually come back up to the normal position and usually stay there. Once or twice it has not pumped back up and if I press it to the floor, it seems like I only have braking at one when (one wheel seems to lock up) and very limited braking. I am hoping to find someone else that has had a similar issue and solved the problem. its strange, because sometimes it asks normal, usually at the first drive of the day and acts up after it been driven a lot.

I have heard of people changing the master cylinder and it not fixing the issue. I would prefer not to start throwing parts and it and replace only the part that is causing the issue. Hopefully someone know what the issue is.

Thanks

 

Bleed the brakes and install new fluid. Shop can do it or you can DIY and just search youtube for how. Not hard but it is helpful to have a helper.
Do that first, may have air in lines or degraded fluid. Then see where you are at. If you still have an issue it is time to look at calipers, rest of brakes components, Mastercylinder/booster

 

Sounds like a bad master cylinder. Air in the lines usually makes for a mooshy and sometimes unpredictable brake pedal. A firm pedal while stopping but falling pedal at stop usually means the master cylinder bore/sealing surface has failed and is leaking fluid. Upon initial stopping, the leak is small enough to not cause an issue, but after holding the pedal at a stop, it bleeds pressure until the pedal falls. It can be from moisture in the MC reservoir causing corrosion in the bore - not always the case but often.