Brake weirdness - pedal drops...

Moderators: Amskeptic, Sluggo

Post Reply
jalabert
I'm New!
Location: Noo Zelun' - left hand side...
Contact:
Status: Offline

Brake weirdness - pedal drops...

Post by jalabert » Mon Nov 11, 2019 11:18 pm

Evening all, long time no visit - something of a winter hibernation.

Just come back from my first roadtrip of the season - bus ran great, used maybe half a pint of oil in 700-ish miles of tough and windy roads. The brakes have though, developed an odd and slightly alarming trait, one that began after a hard emergency stop: when I brake, I have a few seconds of resistance with the pedal pretty high, then it'll suddenly drop by maybe an inch. The time before the drop seems to vary according to how hard I'm braking. There's no loss of feeling - if anything, there's better bite and less effort when the pedal's lower, and I'm not seeing any leakage or anything.

New master cylinder a couple or three years ago, calipers overhauled at the same time, no drops in fluid level. I would have thought if it was a problem with either the booster or the vacuum from the engine I'd be feeling a loss of assistance rather than anything else...

Suggested starting points for investigation very welcome.

User avatar
Amskeptic
IAC "Help Desk"
IAC "Help Desk"
Status: Offline

Re: Brake weirdness - pedal drops...

Post by Amskeptic » Tue Nov 12, 2019 9:38 am

jalabert wrote:
Mon Nov 11, 2019 11:18 pm
Evening all, long time no visit - something of a winter hibernation.

Just come back from my first roadtrip of the season - bus ran great, used maybe half a pint of oil in 700-ish miles of tough and windy roads. The brakes have though, developed an odd and slightly alarming trait, one that began after a hard emergency stop: when I brake, I have a few seconds of resistance with the pedal pretty high, then it'll suddenly drop by maybe an inch. The time before the drop seems to vary according to how hard I'm braking. There's no loss of feeling - if anything, there's better bite and less effort when the pedal's lower, and I'm not seeing any leakage or anything.

New master cylinder a couple or three years ago, calipers overhauled at the same time, no drops in fluid level. I would have thought if it was a problem with either the booster or the vacuum from the engine I'd be feeling a loss of assistance rather than anything else...

Suggested starting points for investigation very welcome.
1. Check brake fluid level in reservoir, top up if necessary.
a) adjust rear brakes specifically and carefully. draw out all adjusters equally (you can count threads looking in the holes) and adjust until the wheels are both difficult to turn. Make a note of the actual friction at each wheel.
b) apply the e-brake firmly and step on the brake pedal firmly a couple of times. You should sense that there is NO drop and it ain't gonna drop. Release e-brake.
c) now *check* the rotational friction of each wheel and report back if one or the other or both slackened noticeably
d) adjust both wheels to difficult-to-turn again, if necessary, then back off each individual star adjuster two clicks, do NOT screw up here, make sure both adjusters are going in the correct loosening direction! If the wheel has not come back to free rotation, adjust each star one tooth looser.
e) apply e-brake 5-6 clicks. Are the wheels sticky-to-turn like they were above? Go make it thus under the car at the equalizer bar. Do not worry if the equalizer bar is crooked, that is evidence of cable stretch, and does not tell you that one wheel is tighter than the other. Get both wheels sticky to turn at 5-6 clicks. At 10 clicks out, the wheels had best be locked up solid. Let me know.
f) Test drive. Is pedal now firm (power brake boost noted) and not prone to dropping away?
Good, yer welcome. If it does still drop away, you need to go in and check. Usually it is a master cylinder failure (don't even tell me that it is "new", that means NOTHING THESE DAYS), but we could have an odd mechanical obstruction like bound up ebrake cross bar.

I personally think you just need a brake adjustment after your gallavanting around.
Colin
BobD - 1978 Bus . . . . . . . . . . . 112,660 miles
Chloe - 1970 bus . . . . . . . . . . . 206,845 miles
Naranja - 1977 Westfalia . . . . . 112,885 miles
Pluck - 1973 Squareback . . . . . . 55,570 miles
Alexus - 1991 Lexus LS400 . . . 78,899 miles

jalabert
I'm New!
Location: Noo Zelun' - left hand side...
Contact:
Status: Offline

Re: Brake weirdness - pedal drops...

Post by jalabert » Tue Nov 12, 2019 5:56 pm

Thanks Colin, looks like a fun weekend full of all my favourite jobs - I have a split CV boot to replace too, so I'll be following the unwritten last step in the CV cleaning / packing process...

Finally, clean up after yourself. This may require burning your house down and starting again.

Seems from what you've written and what I'm feeling is that something's not right in the front / rear brake balance. My rears probably are due an adjustment, but the braking performance feels really good right now - almost as if the fronts haven't been able to engage fully while the rears are correctly adjusted. Everything's even, no suggestion of any binding in the front calipers...

Perhaps the replacement master cylinder is at fault...I have the original, got that rebuilt - might be worth chucking that in and seeing if there's a perceptible difference.

Cheers, Simon

Post Reply