AccordVT

I have searched on this and haven't found an exact answer or an issue similar to this.

94 accord EX 5spd swap
Car is warmed up and at normal operating temp: While driving the idle is fine and motor runs smooth. Intermittently, with clutch pressed or in neutral(no load on motor) while under light but consistent braking pressure the pedal will slightly sink(slowly) and start to feel mushy. A few pumps remedies the sinking for a second. When at a stop and idling the peddle sinks further but still slowly. WHEN EITHER OF THESE BRAKE ISSUES ARE FLARING UP: the idle will bounce up and back down from approx 800 - 1200 rpms. After about 8 seconds the idle returns to normal. Pumping the pedal causes the idle to raise higher and drop faster.

The ABS light is on. I have bleed the brakes and fluid level is correct. My guess is master cylinder (because of the sinking) but the booster would make more sense because of the associated vacuum and idle.

Any help in in determining whether this is booster, cylinder or abs related would be greatly appreciated.

JimBlake

Booster because of vacuum & idle.

But the sinking pedal makes me think you may ALSO have a bad master cylinder.

AccordVT

But would having only a bad master cause a functioning booster to compensate for loss of brake pressure?

JimBlake

Maybe if the MC pushrod somehow gets out of adjustment? I'm not really sure how that would happen.

DOZ5

From what you describe, I think you have two problems, MC and idle which could be the booster or the vacuum check valve. You can check the check valve easily. disconnect the booster hose whie engine is off. suck air from the enine end. the valve should open. then blow from engine end and the valve should be restricting flow (closed). I am not sure where they placed this valve, in the hose or externally.

The pedal sink is hydraulic issue. It is hard to say what it is. You can check it with engine off. Just pump the pedal and it should be very hard and not sink to floor. If it is, but springing back you have air in the system so a good bleeding of the entire system should fix that. If it just sinks to flor slowly but and not springing back you have a leak either external or internal somewhere.

Note: If the fluid is dark, you should flash it and replace it.

AccordVT

I replaced the brake master cylinder and bleed the entire system the other day and the pedal feels better and doesn't get mushy and cause the idle to bounce and search anymore. The idle will still occasionally bounce and search though but much less often after the brake fix. I cleaned my IACV today and it didn't really help. All my vacuum hoses and connections look good. The vacuum check valve (The one in line right near the booster) works (only lets air one way?)

I am thinking there is some vacuum/idle control issue. Any pinpointing or troubleshooting is where I'm at now.

NOTE: After installing the new master cylinder the ABS light is no longer lit.

DOZ5

I think you should disconnect the booster hose (The one with the check valve) from the booster. Start the engine and block the opening of the hose. If the idle change / become better it is the check valve leaking.

Now the question is where the leak in the booster is.

I just replaces this hose on my accord 1999. It was about $25.

do the test and see what's happend.