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Brake Booster? Master Cylinder? Vacuum leak? please help

05 Toyota Tacoma without traction control but has ABS

A few weeks ago I noticed the brake pedal felt mushy. I had my wife step on the brakes with the engine running. I could hear a swoosh of air coming from the rear of the engine near the firewall but more towards the passenger side. I did the following tests:

  1. with engine off pump brake pedal and it built preassure and stayed firm.
    2.with engine RUNNING held foot on brake and turned engine off. The brake pedal moved slowly to the floor.
  2. Check the check valve in the brake booster and its good.

So, I replaced the Brake booster, bleed the air out of the master cylinder and bleed the brakes. Now it feels great for about the first 20min of driving but then goes soft again. and I’m still getting the swoosh of air escaping from somewhere in the same place as before.

I’m wondering if I have a vacuum leak somewhere before it goes into the brake booster? the connection is good where the vacuum hose comes from the engine to the brake booster. OR should I replaced the master cylinder?

Thanks in advance for any help.

Semper Fi to all the veterans out there on this great Memorial Day.

It seems that the master cylinder is the most likely problem.

Semper Fi…

With the engine running, pump the brake pedal rapidly. If the pedal builds pressure and then sinks the master cylinder is defective.


With ABS and an intermittent problem diagnosing is more time consuming. I have accumulated a collection of plugs for master cylinders and even a pressure gauge but for the DIYer you must get a helper to press the pedal with the engine off (to disable ABS) and when the pedal becomes spongy and pressure is held firm open the brake line connections to see if one of them has no pressure. Of course, opening one will cause the pedal to drop if pressure was present so that fitting must be closed, the pedal released and re-applied and held until it becomes spongy again to \check the other port.

Important note; is it possible that an incorrect fluid was poured into the master cylinder?

Also, does the ABS light indicate a problem?

Tester, with the engine running if I keep pumping the pedal it stays mushy and I can hear the release of air near the engine. Only time the pedal gets firm is while the engine is off. But I’ll check that again tonight when I get off work.

Red Knox, when the engine is off I can pump up the pedal till its firm. (3-pumps) When I bleed the master cylinder I ran a clear hose from the two ports up and into the master cylinder. pump the brakes till i had no air. it was difficult. one kept blowing off. When I thought it was good I had someone press down on the pedal at the sametime I was connecting the brake lines.

I just had the brake fluid flushed in december and used toyota fluid. This time I used synthetic DOT3-DOT4 fluid but says it can be mixed with other fluids. I bleed the brakes till I knew I was getting new fluid.

Toyota says to replace the O-ring on the back of the master cylinder that goes into the power brake booster when removing. I used the old one because i didn’t have a new one. Also I could only get three of the four nuts tight on the fire wall inside the cab where the brake booster comes thru. could that be a issue? The brake booster doesn’t move when applying the brakes.

No abs light or check engine lights on in the instrument cluster.

The brake booster has a very strong return spring and with the engine off you must overcome that spring as you depress the pedal to apply braking pressure. With the engine running, depressing the pedal uses vacuum to boost your effort and when the pedal is released the vacuum is vented under the pedal and can be heard on some cars. The “whoosh” when the pedal is released is normal.A faulty booster will; 1 give no boost, i.e., the pedal will be hard with the engine off and starting the engine has no effect. 2 when the engine is running a constant “whoosh” is heard under the pedal due to a leaking vacuum diaphram. 3 the brakes drag due to the booster preventing the master cylinder ram from fully retracting and releasing pressure. ABS makes diagnosis much more difficult since the ABS system has the ability to release pressure. Disable the ABS and drive carefully on an empty parking lot making repeated stops and if the pedal becomes soft and slowly sinks when holding pressure on it there is a very good chance the master cylinder is the problem.

BTW, I & H&S Cos, 3/4, 1969-1970.

Thanks!! The brake booster I purchased came with a master cylinder. Tomorrow I’m going to swap the master cylinders. I’ll report back.

2MarDiv 90’s Mostly Camp Lejune, and Gitmo

A brake pedal should never sink to the floor.
I think you’ll find that the new master cylinder will solve your problem.

By the way, a “shoosh” from the booster heard with your ear near the booster does not suggest a failing unit. You can hear the sudden venting of the rear of the diaphragm. Based on the symptoms, I believe you changed the wrong part. Sorry.

After many years working on cars I seem to recall 3 master cylinders that were bad out of the box. Although rare it does happen. What brand part was installed? Could you have bought a part labelled NEW from a parts house? And was the master cylinder bench bled before installation? And some of us didn’t realize the master cylinder was replaced with the booster. Some of us are accustomed to calling the combination a ‘loaded booster.’ But it still sounds like a master cylinder bleeding down.

And I was drafted in 1968 when pay was $103 monthly. But overseas I could drink all the warm Carling Black Label I wanted for free.

Rod, if I understood the OP correctly, he bought the booster/MC assembly and only installed the booster portion of it.

Excellent point about bench bleeding the MC.

Now it feels great for about the first 20min of driving but then goes soft again.

That’s what she said…

Well, I replaced the Master Cylinder tuesday. The parts I’m using are OEM parts but are used. I did bench bleed the master both times. I still had mushy brakes. I was fed up by this point so I took it to a Toyota Mechanic. They said the whoosh sound I’m hearing is normal and If it was a vacum leak or something else the truck would be running rough and i would get a check enigine light or some codes. I was having a hard time accepting that because I’ve never heard that noise on another car. I asked them why the brakes felt bad? They took it around back and adjusted the rear drums. Brakes feel great now. I’m laughing about it, at least I learned how to repair something new that would have scared me before.

Thanks to everyone for the help.

so, basically the rear drums were the problem the whole time ??