[TeryT]

I'm pretty sure my master brake cylinder is leaking into the booster. Earlier this year I kept having to add fluid on my 2003 V8 (240 K miles), especially when driving in the mountains (because light was coming on), but I could find no apparent leak anywhere. The shop I took it to said it was just the rear pads worn down . . . which didn't make sense, but I had them replaced anyway in July. It seemed like the leak stopped (or maybe I stopped paying close attention) at that point.

Just this past week the light came on again and same thing - no apparent leak anywhere, and no loss of pedal or braking.

I now think the master is very slowly leaking into the power booster.

So has anyone had this issue and has replaced the master cylinder on a 4th gen (V8)? What kind of cost am I looking at?

[MikeinNH67]

There is no vacuum booster on these trucks like what there used to be. These use a motor for pressure so there's no way the fluid is being sucked into the engine, unless I'm mistaken. If you are losing it then there must be a leak somewhere so you should see the fluid under the car or behind the brake pedal.

[TeryT]

Originally Posted by MikeinNH67

There is no vacuum booster on these trucks like what there used to be. These use a motor for pressure so there's no way the fluid is being sucked into the engine, unless I'm mistaken. If you are losing it then there must be a leak somewhere so you should see the fluid under the car or behind the brake pedal.

Yeah, and there's the rub - I've been under it, as well as a mechanic and an oil tech a number of times over the past 4-5 months, and no one sees nutin!

[AuSeeker]

Originally Posted by TeryT

Yeah, and there's the rub - I've been under it, as well as a mechanic and an oil tech a number of times over the past 4-5 months, and no one sees nutin!

As mentioned if the maser cylinder is leaking past the piston there would be fluid coming out the back of the master cylinder, you won't find that leak from under the vehicle, you need to look under the dash where the master cylinder push rod connects to the brake pedal, you my have to pull the rubber boot off the back of the master cylinder or not if you losing as mech fluid as you say it maybe leaking past the boot.

[TeryT]

Originally Posted by AuSeeker

As mentioned if the maser cylinder is leaking past the piston there would be fluid coming out the back of the master cylinder, you won't find that leak from under the vehicle, you need to look under the dash where the master cylinder push rod connects to the brake pedal, you my have to pull the rubber boot off the back of the master cylinder or not if you losing as mech fluid as you say it maybe leaking past the boot.

I've looked at both sides of the firewall and haven't seen anything. I'd think with that much fluid I'd see some evidence, even without pulling the boot off.

The shop said next time the indicator light goes off to bring it in and they'd do a pressure test on the master.

UPDATE: Just checked again and finally see a leak coming down the frame near the firewall - so finally the mystery is solved! The next question is can the master cylinder be rebuilt or better to go with new one?

[MikeinNH67]

Seems you have a trail to follow now. Can you tell exactly where it's coming from? Maybe take a pic of the area to share? As far as I can see you can't buy just part of the expensive assembly but I thought I've read that there's rebuilt units. Would stink to pay a grand for what may be just a bad seal.

I dug around a bit and found these posts. There's probably more.
Brake Master Cylinder Leak
anyone have electric brake booster problems/failure?
2003 v8 4runner ABS Pump Problem! Any Solutions?