The same thing happened to me with 2002 T4R out of nowhere when I was close to home. The truck did not lose any braking ability so I drove it back.
Troubleshooting
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1. I had replaced the accumulator myself about a year ago. It should not have been it. I turned the engine off and pump the brake. I got about 50 times out of it before I drained the accumulator. Good sign..
2. I kept stepping on the brake with the drained accumulator and started the truck. The brake did not sink any further. It ran for 2 minutes and the lights and beep came back along with puffing. This was my old trick to test the MC in my old corolla. Not sure if this was relevant with 2002 T4R. But I took it as a good sign.
3. Repeated Step 1 and got about the same number of pumps. Good sign. That said my accumulator and electric pump were probably okay.
4. I turned on just the accessories to confirm Step 3. 2 minutes in, the pump stopped and the accumulator was charged. At this point, I was sure the pump and accumulator were not my problem. Lights+noise+puffing came back.
5. At this point, I was looking into getting an old entire MC assembly on ebay, but I read that it would need to be re-programmed with my VIN. I could not find any info on how to do that so I went back to analyzing the assembly.
6. I repeated Step 3 and 4 and noticed that the electric pump kept running while the brake was puffing and it darned on me that there must have been too much pressure with the pump constantly on so puffing could have been a way to relieve the pressure.
7. I homed in (googled) on relief valve and pressure sensor and came across this master cylinder pressure switch, PN# 89637-30050. This is a cylinder-shape switch sitting right below the brake fluid reservoir. I ordered one for $320.
8. It'd been 2 weeks since the symptom when the the switch arrived. Armed with that and an extra deep 30 mm socket, I drained the accumulator and replaced the switch. I was surprised that very little brake fluid came out.
9. Finger crossed...I started the truck...NO MORE LIGHTS/BEEP/PUFFING!!!
Here is the picture of the switch
