(Mechanical genius required) Brake pedal spongey even after brake shoe adjust

JooFox

Joe_Horner wrote: »

Very helpful info

Thank you, I think you've covered all bases there so I'll deduce tonight and report back.

Thank you again

adouglasmhor

JooFox wrote: »

Completely agree, I'm a qualified "engineer" and work on flying things with spinning rotors. But I work with qualified "engineers" that I wouldn't trust to change an air freshner in my car

Don't get me wrong, if there's a sludge drying bed or a field drain or a supporting wall holding back tons of unstable clay soil, or decorative beds made on top of contaminated ex industrial land my dad was the man (our local allotments were flooding for years and he came down on his day of walked the land with a surveyors pole and found the problem within an hour and contacted the organisation responsible and got them to sort it.). But he does not know the difference between a spade and a shovel.

He wanted to go to Art School but my Granddad insisted he got a proper job.

Notmyrealname

If there is no leak and no air then the master cylinder seals are shot. It is common for that to happen when you bleed the brakes or the pedal needs pushing down past a normal range of travel as over time, a lip builds up in the bore and when the seals go past that, it can tear them or turn them round the wrong way.

cajef

I find it difficult to believe then an 'engineer' would think that adjusting the brakes would cure spongy brakes.:eek:

lee678

Joe_Horner wrote: »

From your OP you're likely to have 2 separate possible problems:

The fact the fluid was "nearly empty" may or may not be a problem. Was it low before or after you replaced the front pads? And how low were the pads? As disk pads wear the fluid naturally falls in the resevoir. By the time they're worn out it can look pretty low but should come baack up to the "proper" level once they're replaced. If you top it up and then replace them, it will usually overflow as the new pads go in!

If it wasn't looking low because of the pad wear then there MUST be a leak somewhere for it too have gone down. The most likely is probably the rear cylinders as mentioned earlier. If they weep slowly then the fluid gets absorbed by the linings and / or burnt off with the heat of braking so you won't see it until you get the hubs off..

The second problem is the pedal going low. The fact it happened suddenly strongly suggests that it's a master cylinder seal failed. Unless you're talking about a pre-'75 or so car it'll have dual circuit brakes so that a failure in one circuit still lets half your brakes work. An effect of that is that, if a seal fails, the pedal will go much further to the floor but still be "hard" before you completely run out of travel.

About the only other reasons for a sudden change like that are an auto adjuster failing and letting the shoes completely clear of the drums (but then you'd have no problems getting the hubs off), a burst pipe (but then you'd be losing fluid faster than a fast tuing and have a puddle on your drive) or possibly a flexi-hose that's failed and starting balloning under pressure.

You can check the flexis by getting someone else to push the pedal hard while you watch / feel them - it'll be very obvious if they're balloning to that extent.

You'll need to get the drums off to check the rear cylinders and shoes - if you can't shift them then get a cheap 3-leg puller (should get one for less than a tenner) and wind them off with that. Hitting their edges with a big hammer while they're under pressure from the puller helps. There's a risk of damaging the drums doing this but sounds like you're planning to replace them anyway.

One point to note is, once you've replaced the rears, don't adjust them up on the handbrake cable! That will usually prevent the auto adjusters from doing their job and leave you with unreliable adjustment. Slacken the cables off and let the brakes adjust themselves up (usually by pumping the brake pedal) and only adjust the cables once the auto adjusters have worked.

excellent post that should be read again by original poster, can i add, a hose clamp maybe handy if the pedal is still low, you can use the clamp to find out which brake/s are causing the low pedal, if you clamp all four and the pedals still low then the master cylinder is u/s, difficult to fully give help as i havent a clue what vehicle were talking, plus old discs with new pads will also give a spongy pedal till at least theyve bedded in or might never.

MX5huggy

Limey wrote: »

:rotfl:

I have the pleasure of helping scrape up the flying things with spinny rotors when they fall out the sky.

What happened to this one then? Hitting power lines seems a bit risky to me?

http://www.westerntelegraph.co.uk/news/county/9824541.RAF_starts_investigation_after_Chinook_incident_leave_500_Pembrokeshire_homes_without_power/

datostar

Joe_Horner wrote: »

The second problem is the pedal going low. The fact it happened suddenly strongly suggests that it's a master cylinder seal failed. Unless you're talking about a pre-'75 or so car it'll have dual circuit brakes so that a failure in one circuit still lets half your brakes work. An effect of that is that, if a seal fails, the pedal will go much further to the floor but still be "hard" before you completely run out of travel.

I favour that one. Failed servo seals can cause a sudden loss of fluid too and there's no visible leak because the brake fluid gets sucked into the engine.

Robisere

No one has asked the OP the most important question; what make and model of car?

And I am an engineer. I was an aircraft technician, then a motor engineer and workshop foreman.

lee678

Robisere wrote: »

No one has asked the OP the most important question; what make and model of car?

And I am an engineer. I was an aircraft technician, then a motor engineer and workshop foreman.

i have two posts back
"difficult to fully give help as i havent a clue what vehicle were talking"

Joe_Horner

Damn there's a lot of ex-ATs* round here!





*did type blackies but then thought that might get misundersood