Master Cylinder, Seal, or Booster

[PatrickStapler]

I guess with 300K miles I should expect a new challenge on a frequent basis. Anyhow...below is a picture of the brake booster just below the master cylinder. Appears to me that brake fluid has bubbled the paint on the booster housing. The parking brake light came on signaling low fluid level I presumed since the brake was clearly not set. I checked the level a few days ago and it is low at the MIN line. I didn’t notice the situation on the booster until today when the truck started to run really rough at speed like a vacuum leak. So I popped the hood to look for loose hoses since I just replaced two fuel injectors a month or so ago and some vacuum lines in the process. The engine idles smooth and seemingly normal. Since I am by far no expert on these trucks, my question is has the MC gone or going bad, bad seal only, bad gasket at the booster or what? Can any of these cause a vacuum leak at this interface?

 

[sleepydad]

I guess with 300K miles I should expect a new challenge on a frequent basis. Anyhow...below is a picture of the brake booster just below the master cylinder. Appears to me that brake fluid has bubbled the paint on the booster housing. The parking brake light came on signaling low fluid level I presumed since the brake was clearly not set. I checked the level a few days ago and it is low at the MIN line. I didn’t notice the situation on the booster until today when the truck started to run really rough at speed like a vacuum leak. So I popped the hood to look for loose hoses since I just replaced two fuel injectors a month or so ago and some vacuum lines in the process. The engine idles smooth and seemingly normal. Since I am by far no expert on these trucks, my question is has the MC gone or going bad, bad seal only, bad gasket at the booster or what? Can any of these cause a vacuum leak at this interface?

I can't say I have had that happen but I have replaced the master piston with their piston kit.

see at the bottom on this post
http://www.toyota-4runner.org/3rd-gen-t4rs/247329-2000-limited-sleepydad-build-2.html#post2885174

the brake booster has that vac line from the manifold vac zone. you can test the booster by pressing on the pedal then starting the truck, the pedal should go further indicating your getting help from the booster. also I'm 100% sure some vids out there showing the exact test.

it almost sounds like the seal at the end of pistons on the master is leaking and the vac inside the booster is sucking brake fluid into the booster? it's really easy to replace the main piston. make sure you also get the 2 seals for the reservoir, just o-ring things but mine were also leaking just a bit. you will have to bench bleed the master. it's super easy and their are vids out there, it just kind of makes a mess. I put plastic down and

here is the master for a 2000
https://partsouq.com/en/catalog/gen...g$&vid=0&cid=2&uid=265715&q=JT3HN87RXY0283787

and this one is the booster
https://partsouq.com/en/catalog/gen...A$&vid=0&cid=2&uid=265719&q=JT3HN87RXY0283787

 

[PatrickStapler]

Thanks for the links sir. Seems pretty straight forward.

 

[nevada]

the master cylinder is leaking. the internal cup has failed. its possible that you now have brake fluid in the booster and it is damaged also.

I know some people will say to just rebuild it, and you might be able to and get it to work. but brakes are pretty vital, if it goes wrong, things can go REALLY wrong.
I suggest just replacing the MC, along with the booster if needed.

 

[evamoly]

Hey all,

I have a 1999 SR5 4wd with 185k miles and a similar issue -

Three weeks ago had new front pads, rotors, hard lines and caliper hoses replaced at a Toyota specific shop. They did a system flush and bleed.

Three days ago while descending a steep paved grade at low speed I lost almost all braking power but was able to slow it enough to pull over and stop by pressing with all my weight on the pedal. I popped the hood and noticed the fluid in the reservoir was pretty discolored, definitely not like I would expect three week old fluid to look. I let the car sit with the engine off for ten minutes and regained enough braking power to get home. I did the booster test with the engine off, pumping the pedal until it was firm, then sank to a normal position when the engine started, so I don't think it's the booster.

My suspicion was that they did not change the fluid and that the contaminated fluid boiled when the system heated, then when it cooled i regained some power.

I called the shop who did the work and explained the symptoms, and they responded that the master cylinder had blown and it would be a $480 fix.

Does this sound right? The photos included show a possible leak, and also the hose going to the engine not entirely seated at the clamp.

 

[Bad Luck]

Hey all,

I have a 1999 SR5 4wd with 185k miles and a similar issue -

Three weeks ago had new front pads, rotors, hard lines and caliper hoses replaced at a Toyota specific shop. They did a system flush and bleed.

Three days ago while descending a steep paved grade at low speed I lost almost all braking power but was able to slow it enough to pull over and stop by pressing with all my weight on the pedal. I popped the hood and noticed the fluid in the reservoir was pretty discolored, definitely not like I would expect three week old fluid to look. I let the car sit with the engine off for ten minutes and regained enough braking power to get home. I did the booster test with the engine off, pumping the pedal until it was firm, then sank to a normal position when the engine started, so I don't think it's the booster.

My suspicion was that they did not change the fluid and that the contaminated fluid boiled when the system heated, then when it cooled i regained some power.

I called the shop who did the work and explained the symptoms, and they responded that the master cylinder had blown and it would be a $480 fix.

Does this sound right? The photos included show a possible leak, and also the hose going to the engine not entirely seated at the clamp.

That is not fresh fluid. They definitely didn't flush your brakes. New brake fluid is clear and that looks way to dark. The color looks like they tried to suck out as much old brake fluid as possible from the reservoir and then filled with new fluid. I would take it back to them to refund you the money. A brake flush is really easy with a few basic tools and a helper to push the brake pedal.

Looks like your vacuum hose isn't seated fully on the booster. I would do my own brake flush and push the vacuum hose on further and move the clamp to the correct spot.