View Full Version : Pedal sinking to the floor
I just changed the pads and fluid on my Astra. The car pulls up fine, although if I hold pressure on the brake pedal while stationary, it slowly sinks to the floor.
Does this mean the seal in the master cylinder is rooted and is allowing the fluid to seep past? There are no visible leaks anywhere in the brake system.
If its the master cylinder, is it worthwhile rebuilding it, or just easier to go with a new / remanufactured unit?
Mick_MY02
12-04-09, 05:39 PM
Most likely air bubbles in the fluid.
Rebleed from scratch to fix.
Madhatr
12-04-09, 06:27 PM
Does it actually travel all the way to the floor, or just moves down and stops?
Switch the car off.
Pump the brake pedal 4-5 times to get rid of the booster assistance.
Then hold down the brake pedal.
If it stays firm, master should be fine, if it sinks to the floor then, its bypassing and could be a seal or a relief thats faulty.
It's a _strong_ possibility you have damaged the master cyl seal accidentally during the fluid change/bleed process.
the 'why' is simple enough - during normal use the pedal (and master cylinder plunger) travel is only perhaps 1-2 inches. So any microscopic debris is pushed to the end of that travel. Over time it builds up (additionally the seals wiping along that first 1-2 inches keep it clear of any corrosion, however slight, but it can build up at the end that it never travels over).
Then someone comes along and bleeds the brakes, which involved pushing the pedal all the way to the floor during each bleed cycle (and there are of course a handful of cycles each corner till you get fresh fluid through there).
And that very bleed process then pushes the seal over the debris and damages it!
And for hte record - I learned this the hard way, so I'm not putting any sort of shit on anyone for doing it.
To avoid this, you can opt for a vacuum or pressure bleed that doesn't require the pedal to be pushed at all (but does require a little bit of tooling - which can actually be diy). The other cheap option is to put a lump of timber under the brake pedal to limit it to 2 inches of travel (or whatever amt it sees in normal use) so that when bleeding, it'll hit hte timber, and the assistant can hold it there whilst the bleeder is re-tightened for the next cycle/go around.
Last but not least, you can run a tight fitting hose from the bleeder into the top of a vegemite (or similar) jar. have the hose reach through to the bottom of hte jar, and have it 1/4 full of brake fluid (so it can't suck air back in there). then all you need to do is crack the bleeder and push the pedal down 2 inches, then release, repeat until fresh fluid is in the jar. you can safely let go of the pedal, and get out and nip the bleeder back up - making it a one man job.
If the master cylinder is indeed fucked - don't waste money on new seals, as they almost never 'work' since the cylinder will be corroded/damaged. The 'best' option if you care for it, is you can get the master cylinder bored and sleeved with a stainless steel insert. This _will_ take a new seal set and being stainless never corrode - meaning that when those seals eventually go, you can get away with just replacing the seals. Given the results I've had with this (on both master cylinders and wheel cylinders on drum braked rears) I actually prefer the stainless sleeve over a new as cast/machined bore of most new master cylinders.
What jmac says is der troof! I have been there on more than one occasion with a couple of my cars.
i also agree with jmac , ive also encountered this problem.
Not sure 100% if its the master cylinder. With the car off removed the air out of the booster and the pedal is firm. I'll try bleeding the brakes again although last time, the fluid coming out was nice and clear, free of bubbles.
Would a faulty master cylinder cause the pedal feel to sink slightly then first up? Or will the pedal sink to the floor with faulty seals in the master cylinder.
Not sure if Im chasing the wrong thing.
It's a real tough call to make - there might be pitting at one point on the master cyl bore and it starts to seal up later (somewhat unlikely as the pitting tends to happen at the end of the travel that is never wiped clean, not the top, but who knows.
You also might have a proportioning valve doing something wacky.
I'm assuming (possibly wrongly) that you only changed pads at one end. If so, try bleeding the other end.
About the only other thing I can enquire about is the bleeding method - the way I was originally taught (which is incidentally not the best way, so if that's what some apprentices were taught, god help everyone else) was to pump the pedal till it's firm push really hard, then get an assistant to crack open the bleedr, and you push and hold on the floor till they tigten it. It works 'quick' but the problem is it aerates the fluid. Any chance this is happening?
To avoid it there are two general ways:
with 2 people, the guy on the bleeder calls out 'down' and cracks the bleeder and the guy in the car pushes the pedal to the floor (or the bit of 2 by 4 to prevent full travel and aforementioned damage) and holds there till the guy on the bleeder nipple nips it up and says up. raise the pedal and repeat, checking fluid level in master cyl reservoir about every 7-8 cycles.
If alone - get a firm fitting hose and put on the bleeder nipple, and crack the nipple just 1/8th of a turn or so. put the other end of the hose into a jar which is 1/4 full of brake fluid (with hose in the fluid, so it can't suck air back up) - to hold the hose firm, drill a couple of holes in the lid (one to let air out as it fills, and the other fit a grommet to it and feed the hose through. now you have a sort of fluid one way valve. simply push the pedal to the floor (or the 4x2 block of wood) and then release and repeat 7-8 times, then close the bleeder nipple and top up the reservoir and repeat, then move to the next corner. simple effective bleeding and can be done solo.
Other things to look for - it could 'feel' hard with no vac in the booster but is it actually providing adequate braking (only test this on a deserted road with massive run off and don't exceed 40km/h if you disconnect the booster. You might well find that the pedal feels hard but isn't actually stopping the car (it might hold it stationary when at rest and 'fool' you)
Also look at stuff like wheel bearings, if they are stuiffed and the hub/disc can wobble at all, they knock the pads off the disc a little (which is a bad thing despite some old wives tales) and then you have to take up that travel just to get any actual friction/braking effect.
After typing all that, I have to be honest, if there's any doubt about the master cylinder, I'd personally be very much inclined to replace it (and go the stainless sleeve option if available) - it's not something to be gambled with.
Jmac, thansk for all your info, in this thread and my LPG thread! I cant thank you enough.
In relation to the master cylinder, originally I thought it was the master, athough the pedal does not sink right to the floor. It will sink and then brake. The car stops fine and even when the pedal drops low, it still stops the car.
I changed front and rear brake pads and bled all brakes.
Ive got a new master cylinder coming so Im going to replace that (its only costing around $150 for a new one from overseas), bleed it again and see how the pedal feels.
I was reading up the workshop manual (its a Astra) and it says that to reduce the pedal travel for the brake, bleed the rears. Ive bled all brakes about 3-4 times with the same result.
The method I use is the one man method. I bought the kit from Repco which is similar to what you describe to make up. Im certain its bleeding well and getting the air bubbles out because the line coming from the bleeder is clear of all bubbles and air.
Think Ive found the problem. Drivers side caliper has 3 out of 4 pistons seized and passenger side has 2 out of 4. Time for a rebuild. Or should I go new (around $400 each new :()
A few ifs involved - are you _sure_ they are seized? They'd be showing piss poor pedal if so, at practically all times, as they'd 'stick' and lift off away from the disc as soon as pressure was off, requiring a lot of pedal travel to push back out and against the disc.
If it truly is the calipers, and you have the time to spare, you might as well strip them down and check the condition. If the pistons themselves are scored or damaged, you'd have to look at either replacement pistons, or stainless sleeved or just stainless pistons. If they are just generally 'sticky' then cleaning up the pistons with 1000grit wet and dry, mounted on a leather belt/strap and _heavily_ lubricated with trans fluid is an option and then new seals of course. use rubber grease (not ridiculous amounts) to lubricate/install the seals (or you can also actually use brake fluid itself, since it's compatible 100% with said seals.
If you are in _any_ doubt about the diy options, then there is no doubt, just go the new calipers.
Sorry got a bit excited there, I had two brake dramas.
The Astra, I changed the master cylinder and seems to be fine. Pedal travel is a bit low for my liking although I guess there is only about 150mm of pedal travel in total anyhow. I'll rebleed and check the calipers front and rear to see if they are ok. Is the best way just to take the caliper off and then press on the brake pedal slightly to see if they come out?
The brakes previously on the Astra were hard and it looked as though the fluid hadnt been changed before so the brake feel is probably alot different now but perfectly normal.
My second problem and what I was referring to were the brakes in my Hilux. Changed the pads, rear drums, shoes, wheel cylinders and fluid and what I would get with the Hilux is braking (and it would pull up ok) although it would then hit a dead spot like a block of wood was behind the brake pedal.
Tried swapping master cylinders, brake boosters etc to no avail. Decided to take the pads out of the front and then clamp the brake line to one side at a time and slightly press the brake pedal to see what was happening.
End result was what I posted above, one piston working (out of 4) on the drivers side and two on the passenger (out of 4).
Sorry for the confusion. With the Hilux calipers, I'll be getting the local brake place to overhaul them and then I guess replace if there is any doubt with them.
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