Sinking brake pedal

Post by **PSL184** » Mon Aug 04, 2008 12:09 pm

I replaced the M/C for the MOT this year (June). The tester said the brakes were good for a Minor so I was pleased. Yesterday, I jumped in the car for a spin and the brake pedal shot straight down to the floor! After I pumped the pedal a few times I got brakes but the pedal doesn't feel like it has any resistance. I checked the level and its not moved from where I topped it up to. I can't see any leaks anywhere and no obvious signs of a proble. The pedal was firm the last time the car was driven so any ideas whats gone wrong?

Post by **Rob_Jennings** » Mon Aug 04, 2008 2:19 pm

pumping the pedal has to be air in the system

assume one of the wheel cylinders may have given in, or check the flexi hoses at the front or the single one at the rear.

any air in a system that was good would suggest a leak.

Post by **PSL184** » Mon Aug 04, 2008 2:23 pm

Hi Rob. I agree that it has to be air in the system but I have been over everything and there are no leaks. I haven't bled the system yet as I didn't want to introduce and more variables until I found the problem. Do you think it is possible that air may have been trapped somewhere before and has now worked its way into the fluid somehow?

Post by **Rob_Jennings** » Mon Aug 04, 2008 2:30 pm

no, any air locks result in a soft pedal no matter where they are as they add compressible volume to the whole system.

if there is air there now you have a failure somewhere? if all the wheel cylinders are good with nothing in the drums to indicate a leak then the flexi hoses are likely cause, they can split in all sorts of odd ways (allowing air in with minimal sign of leak, acting as valves if they collapse .....)

Post by **PSL184** » Mon Aug 04, 2008 2:33 pm

OK thanks Rob. I'll re-visit the flexi's..... Rich

Post by **bmcecosse** » Mon Aug 04, 2008 7:17 pm

If it's not a leaking slave cylinder or pipe - then it must be failed master cylinder. Unless you have a servo ? They can fail and start leaking the fluid away into the inlet manifold!

Post by **PSL184** » Mon Aug 04, 2008 7:45 pm

No servo - just completely standard brakes. Fluid level has definately NOT gone down in the M/C and this was brand new in June from a proper supplier (not off fleebay or autojumble etc...). I need to look closely at the flexi's, as Rob has pointed out. Someone else has told me they had a similar problem with the rear one.... Thanks all. I'll update when I get time to have a proper look.

Post by **bmcecosse** » Mon Aug 04, 2008 7:49 pm

Flexi will not give that effect unless it's actually leaking - and you say no leaks/no fluid loss. So - i'm afarid it IS the master cylinder. Warranty Claim ?

Post by **PSL184** » Mon Aug 04, 2008 8:48 pm

Once I have satisfied myself that it's nothing else then yes, I will be approaching the supplier. Just seems strange thought hat it was working fine and has done no more than 200 miles since fitted. I need to get time (probably weekend) to strip and check everything else. Rear cylinders and shoes were all new at the same time as the M/C so I guess front end is suspect but still after a second look just now, I cannot see any leaks. How frustrating !!!

Post by **les** » Mon Aug 04, 2008 10:13 pm

I get the same after leaving it for a day or so, in fact I renewed the m/cylinder only to get the same; a couple of pumps and it's ok until the next time, no leaks etc dismantled the 'old' cylinder and everything good, seals and bore. One thing I haven't done is lower the fluid level, looks like I have been running with it at the bottom of the cap, whereas it should be 1/2 inch below, whether it matters I'm not sure!

Post by **aupickup** » Mon Aug 04, 2008 10:49 pm

also check bleed nipples if they are repro ones

Post by **kennatt** » Tue Aug 05, 2008 8:10 am

before you start to strip just check all the brakes for correct adjustment,just a chance. You could also clamp up each flexi in turn and try the pedal if you get a hard pedal with one of the flixis clamped you have found which brake is causing the problem and cut down on the investigation

Post by **PSL184** » Tue Aug 05, 2008 8:24 am

Thanks for the tip

Post by **bmcecosse** » Tue Aug 05, 2008 10:30 am

I don't like 'clamping' flexis - they often start to swell/fail shortly later - just at the clamp position! Better to just pull the wheels off and inspect. However - kennatt does have a good point - if the brakes are badly out of adjustment - then first press of the day will cause the shoes to move up to the drums and can make for a long stroke. The little seal should then hold them in place - and all other strokes should be normal unless the car is left for a while, whereupon the brake shoes will relax back again. So - if the second push is hard - it's maybe just adjustment that's required!

Post by **kennatt** » Wed Aug 06, 2008 8:04 am

[quote="bmcecosse"]I don't like 'clamping' flexis - done it for years never had a problem it's the first thing a garage does with brake problems there is a special clamping tool they use like a pair of mole grips with a wide type of jaws ,but if you use normal ones and are worried about damaging the pipe just place two strips of hardwood between the jaws,and this will spread the point at which the pipe is closed off you don't need much pressure just nip it up.I'ts without doubt the quickest and most acurate way to isolate a fault

Post by **les** » Wed Aug 06, 2008 9:38 pm

These devices have curved jaws to give a less brutal grip, but imo bm has given good advice, especially if the age of the pipe is a factor.

Post by **kennatt** » Thu Aug 07, 2008 5:13 pm

I take on board all that bmc and les says but if the pipes are that old that they can be damaged they should be changed anyway wouldn't you say.

Post by **MoggyTech** » Thu Aug 07, 2008 7:37 pm

kennatt wrote:I take on board all that bmc and les says but if the pipes are that old that they can be damaged they should be changed anyway wouldn't you say.

It's a sad fact that the brakes on most cars on the road, are the least likely item to get regular maintenance. Yet they are the most vital component on a car. Even with brake fluid changes every two years, rubber flexi hoses should be replaced every eight years MAXIMUM. Proper hose clamps will not damage a good hose that is fairly new, but they will wreck an older hose that should have been replaced anyway.

Single circuit brakes, take NO chances. That's why brake parts are among the cheapest bits to buy for most cars.

What I do to minimize fluid loss when repairing brakes, is I made up short pipes with soldered ends, and have old cylinders that are blanked off inside. So depending what I am working on, I can blank off the circuit.

Sinking pedal first thing in the morning is probably just shoes needing adjusted up. The little hat valve in the master cylinder will only hold the 8LBs pressure for a few hours if the car is sitting still.

Post by **les** » Thu Aug 07, 2008 10:03 pm

they should be changed anyway wouldn't you say.

I would, but I'm sure it's been unwittingly done on old pipes. I think they are date marked now.

Post by **bmcecosse** » Sat Aug 09, 2008 5:15 am

All you need to do to prevent 'fluid loss' is to put a thin plastic bag over the master cylinder filler - and then screw the top on over the plastic. No air gets in and no fluid gets out - no more than a few drops anyway!