Intermittent weak brake pedal

[snydmax]

Wife came home today complaining of an intermittent weak brake pedal... told me today and mentioned it’s happened before.

Symptom: little to no pedal... let off and push again and it feels and stops normally. She thought it mostly happens when slow rolling like stop-and-go at a light.

In my test drive it does feel like the pedal may be a little soft but I could replicate what she described... maybe I should get a job at the dealer [emoji854]

I’m thinking to flush the fluid, maybe suspect the master cylinder... booster.

Hoping the 3rd gen family will pipe in if they have any thoughts or have had similar issues


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[FD7683]

Wife came home today complaining of an intermittent weak brake pedal... told me today and mentioned it’s happened before.

Symptom: little to no pedal... let off and push again and it feels and stops normally. She thought it mostly happens when slow rolling like stop-and-go at a light.

In my test drive it does feel like the pedal may be a little soft but I could replicate what she described... maybe I should get a job at the dealer [emoji854]

I’m thinking to flush the fluid, maybe suspect the master cylinder... booster.

Hoping the 3rd gen family will pipe in if they have any thoughts or have had similar issues


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My brake pedal has a longer travel than usual but bites very hard when it grabs. I would maybe flush the brake fluid first if its spongier than usual. Lots of people snag good boosters at the scrapyard for cheap.

 

[snydmax]

Would a bad booster make a soft or a stiff pedal?


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[FD7683]

Would a bad booster make a soft or a stiff pedal?


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I don't think it would be. A bad brake booster could stiffen the pedal or something, not sponge it up (well thats what I read on Google, I have not dealt with one). It could be your master cylinder going out. Hold the brake pedal and see if it loses pressure and goes down. If not, maybe flush the fluid and see what happens.

EDIT: I didn't read it carefully. A bad booster would stiffen the pedal.

 

[spartacus]

Checking the brake lines for degradation and bulges would be a good as well.

 

[JohnMc]

I'm just not entirely sure what symptoms are being described. Little or no pedal means.. what? No pressure and the pedal goes down and down and no braking happens? Or the pedal goes down, you feel resistance and push on the pedal, but very little braking happens as a result?

The check-valve on the vac line to the brake booster got 'sticky' on mine - mostly in vey cold weather. In that case, you have no vac boost assist, and the pedal is rock hard, and it takes a LOT of pedal pressure to get any braking. That braking system is NOT set up for non-assisted braking at all.

Alternatively - it sounds like in this case you have a firm pedal, but occasionally the pedal goes further down than normal, but the brakes work 'relatively' well once you get the pedal down far enough? That could be pad knock-back on the front wheels - if you have something a bit loose (usually wheel bearings) the rotor can wobble a bit while rolling and push the pads back further than they should go (they self adjust by the rotor runout pushing them back off, plus some rebound in the rubber seals). After the first push, they're more or less back to normal, second push they work normally.

 

[FD7683]

Checking the drum brakes for drag when rotating would help too. I adjusted mine recently but really didn't solve my low brake pedal. For some people it did help adjusting the drums.

 

[JohnMc]

Those normally get adjusted when you use the parking/hand brake. If they're a bit too loose (and the adjuster isn't stuck) then it will click a tiny increment tighter. Quote often people with automatic transmissions never use the parking brake, so they just end up getting looser and looser.

 

[snydmax]

I'm just not entirely sure what symptoms are being described. Little or no pedal means.. what? No pressure and the pedal goes down and down and no braking happens? Or the pedal goes down, you feel resistance and push on the pedal, but very little braking happens as a result?

The check-valve on the vac line to the brake booster got 'sticky' on mine - mostly in vey cold weather. In that case, you have no vac boost assist, and the pedal is rock hard, and it takes a LOT of pedal pressure to get any braking. That braking system is NOT set up for non-assisted braking at all.

Alternatively - it sounds like in this case you have a firm pedal, but occasionally the pedal goes further down than normal, but the brakes work 'relatively' well once you get the pedal down far enough? That could be pad knock-back on the front wheels - if you have something a bit loose (usually wheel bearings) the rotor can wobble a bit while rolling and push the pads back further than they should go (they self adjust by the rotor runout pushing them back off, plus some rebound in the rubber seals). After the first push, they're more or less back to normal, second push they work normally.

“Little to no brake pedal” symptom in this case means that the pedal is soft (not stiff/hard) the brakes do grab eventually, though I’m not sure I have actually experienced the intermittent issue my wife is describing.

I.e. pedal is not firm, but the opposite

Suspect MC after some research though the rotor wobble leading to caliper push-back seems worth investigating...

we use e-brake every time it’s parked, so I doubt it’s a drum adjustment issue. I think I’ve finally even trained the little lady to do this 90% of the time


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[FD7683]

You could start off by bleeding all 4 lines first and see what happens. I remember bleeding the system off mine for the first time and it got rid of all the sponge.

I would suspect a bad master cylinder if its still spongy after the job (knowing there is no air in the lines) and if it doesn't hold pressure.

 

[MaineRunna]

Clamp off all three brake hoses with needle nose vise grips or something equivalent and see how the pedal feels with engine running. Rock hard pedal means problem is at the wheels, spongey pedal means master cylinder or hydraulic leak.

 

[snydmax]

Ordered an MC and TBU goodies... will try this before digging in. Pulling and rebooting the cv’s and doing the ubj’s cuz, rabbit holes! Gotta put that diy labor savings to good use!!!


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[diverscale]

you symptoms resembles bladly bleeded brake lines.

Often have to bleed long and go through much oil to get all the air out. I use a transparent tube at the brake's bleeder nipple to monitor until there is NO air bubble getting out. Start with farter away brake from the brake booster.

Like I said, might take longer than you think to take all that air out, but still, this is the easiest way out to begin with.

Also worth looking if your rear brakes grabs when applying the handbrake.