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Brake booster question

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J

JCH13

Diamond Member
I've got a...

1991 Miata
turbocharged
no idle air control

The symptoms...

excessive slop in the brake pedal before brakes engage
noticeable, but not excessive, pedal force required to slow/stop car hard
if i pump the brake pedal a few times in the 'sloppy' area of it's travel the engine idle speed increases

What I've done...

two new front calipers (unrelated)
full brake system flush, improved brake feel a little

What could be causing this...

Bad brake booster?
Blown check valve somewhere? (is there one?)
Something completely unrelated?

Looking for input.
 
if i pump the brake pedal a few times in the 'sloppy' area of it's travel the engine idle speed increases
Click to expand...

This is normal as the brake booster works off vac so it demands more from the engine which intern acts like a vac leak which is like opening the throttle just a tad but also it leans the idle mix out just a bit so it "idles up" just a little...

As for the "sloppy" brake pedal to me it sounds like possibly a bad master cylinder or maybe just air in it...? Did you by chance run it dry when changing the front calipers...? If so it may have just sucked air in the system and needs to be bleed properly... Also it could be an issue with the rear brakes as this is what sets pedal height and can make the pedal feel spongy but if its just adjustment of the rear brakes then pumping the pedal should make it come up and feel firm if not then it could be air in that end...? Need a little more detailed info...😉
 
yeah i gotta agree w/ Bartman. it sounds like you still have some air in the brake lines. did you flush and bleed the brake system by yourself? not that it isn't doable, but its a royal PITA b/c you have to be in the engine bay to keep the reservoir topped off (to prevent more air from getting into the system), you have to be in the car to pump the brake pedal to flush/refill the lines, and you have to be under the car to open and close the bleed valves on the calipers.

i feel silly even suggesting that the brakes might not be bled well enough knowing full well that you know your way around a car. perhaps you just rushed through it...
 
If your engine RPM's are changing considerably-chances are your power booster is leaking.
Though you should hear it leaking (very slight hiss) as the pedal is depressed.
 
Bart/Sunny:

I am very confident that there is no air in the brake lines because I did the system flush with two other people. One guy on the pedal, one guy watching the reservoir, and me operating the bleeder valves. I suppose it is possible that air got in if something really weird happened. I should describe the pedal feel a little better: the pedal takes essentially zero effort in the 'sloppy' part of its travel, then it immediately becomes very firm. There's no 'squishy' or 'spongy' feeling to it. Just a bit of zero-effort travel, then a solid brake feel.

5150MyU:

Would there be a way to test this without running the engine? The car is in a garage now in storage without an easy way to run it.
 
Is the pedal assembly itself worn out, and is perhaps not actuating the master cylinder until the slop in the pedal has been taken up? Kind of like my clutch pedal was, if you remember.
 
Possibly, I'd have to get under the dash and check it out. Will put on my lengthy to-do list 😛
 
Along with checking the slop, have you checked the vacuum hose leading to the vac. booster for cracks? Almost sounds like you have a vacuum leak somewhere in the system.
 
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