Weird Brake Pedal

[Bailes1992]

I've noticed the brakes on my Wife's Daica Logan feeling a bit poor lately. While the brakes worked, there seemed to be excessive travel in the pedal and a lack of modulation.

The car has discs up front and drums in the rear.

I replaced the rear brakes 6 months ago as one of the brake shoe linings became detached and caused some interesting issues.

The front brakes appeared to be original but the discs were still within specification and there was meat on the pads. Quite amazing after 110,000miles.

Yesterday I made the decision to replace the brakes up front. I stripped the old brakes off, cleaned everything thoroughly and installed a new set of R90 spec Bosch front discs and pads.

I then popped the rear drums off, cleaned the slight lip on the inner edge, thoroughly cleaned the internals, lubricated what was required and buttoned all back up.

After all this I flushed a pint of fresh brake fluid through each corner using a pressure bleeder. The old fluid was pretty nasty.

But still, the pedal seems wrong. This is what we are experiencing...

When the engine is off, the brake pedal when pumped once or twice is rock hard.

When the engine is running, the brake pedal sinks to around 1/2 way, then slowly sinks to 3/4 of the travel over 10 seconds or so with constant pressure.

When driving, the first 1/2 pedal doesn't do a great deal. Maybe some very slight braking. Between that 1/2 way to 3/4 way the brakes can be somewhat modulated.

If you anchor on, the car stops very well. Enough stopping power that the ABS kicks in.

My thoughts currently are as follows...

There can't be air in the system as the pedal is rock hard with the engine off.

If the master cylinder was failing, I'd expect a sinking pedal when the engine was off also.

Is this a brake booster issue?

 

[Jetronic]

bleed the abs with a diagnostics tool before anything else. The brake booster can't make the pedal sink there's a rod going through that directly acts on the master cylinder.

 

[Bailes1992]

bleed the abs with a diagnostics tool before anything else. The brake booster can't make the pedal sink there's a rod going through that directly acts on the master cylinder.

Surely this would be evident with the engine off though? I need to find out if the Pyclip software I have is capable of activating the ABS pump while bleeding I guess.

 

[Lubener]

I doubt there is air in the system. Air will not enter unless the system has been opened to replace a component or you let the master cylinder run dry during your bleeding process.. I am concerned that you only get 1-2 power assisted pumps after shut down. I would expect 3-5 power assisted pumps. I would atleast check the booster's check valve with that. I do not believe that is your actual problem.
Are you adjusting the rear drums with a slight drag?

 

[Jetronic]

no, don't thnk you can easily overcome the lack of aid from the brake booster. But in your case it's old fluid in the ABS i suspect, not air.

You don't need to activate the pump while bleeding the brakes, you need to press hard on the brake pedal, and tha abs will pump fluid and air back to the reservoir

 

[Jetronic]

I doubt there is air in the system. Air will not enter unless the system has been opened to replace a component. I am concerned that you only get 1-2 power assisted pumps after shut down. I would expect 3-5 power assisted pumps. I would atleast check the booster's check valve with that. I do not believe that is your actual problem.
Are you adjusting the rear drums with a slight drag?

The rear brakes are self adjusting, all you need to do is get them close enough. if the adjustment works of course.

 

[Bailes1992]

no, don't thnk you can easily overcome the lack of aid from the brake booster. But in your case it's old fluid in the ABS i suspect, not air.

You don't need to activate the pump while bleeding the brakes, you need to press hard on the brake pedal, and tha abs will pump fluid and air back to the reservoir

That's interesting. Always thought you had to activate the ABS pump with a diagnostics tools while bleeding to remove air.

Is that with the engine on or off?

 

[Jetronic]

Engine on I think. Did it a million times and can't remember now lol. but do it wrong and the procedure doesn't start

 

[CharBaby]

You may have a one or more of the hydraulic brake lines on its way out. Check & feel with your hand for expanding while someone presses on the brake pedal.
Or your brake master cylinder is giving you some issues.

 

[Bailes1992]

The car isn't 6 years old yet so doubt the hoses are bad. They certainly look okay.

Spent a little bit of time today doing some research. It really is pointing to master cylinder failure. Apparently one big symptom is contaminated brake fluid. This is what I got out of the car yesterday...

I think I'm going to try bleed the system one more time one dry evening this week. If that doesn't yield any results then I'm going to have to order a master cylinder.

 

[mk378]

The pedal sinking while held with steady pressure is either a leak or a bad master cylinder. If the fluid is contaminated with an oil-based substance, all the rubber parts will swell up including the seal on the reservoir cap which is a good way to diagnose.

 

[Craig in Canada]

At any time during your recent adventures (e.g. when the rear linings delaminated) did the brake pedal ever travel significantly further than in normal use?

The first thing that comes to mind from your description is a master cylinder seal issue. The biggest pointer there is the pedal falling while your foot rests on it. Strong panic braking is also consistent of that because it's a sudden spike in pressure and a slightly damaged seal will still hold. Slowly falling pedal is a peak somewhere - oozing from the piston, leaking from a hose, or leaking past the master cylinder seal. While technically it could be an air or fluid "leak" past whatever seal it is, air isn't going to suddenly compress a little easier while you're holding your foot on the pedal, and a big bubble or air would give you panic stop issues too.

Something I don't feel I see mentioned enough - as cars age the master cylinder bore gets polished in the range where it is most heavily used. This makes it smooth and slightly larger diameter. Past that point the diameter goes back to original and there could be surface imperfections. Grind the seal past that, like during bleeding or some weird brake failure that causes the pedal to fall dramatically, you can now get a little tear in the master cylinder piston seal, for which there is no fix but replacement.

I see lots of bleeding instructions, including BMW's own, which say "pump the pedal to the floor". I specifically place a brick or block of wood under the pedal to keep it in normal operating range on my vehicles.

 

[Bailes1992]

At any time during your recent adventures (e.g. when the rear linings delaminated) did the brake pedal ever travel significantly further than in normal use?

The first thing that comes to mind from your description is a master cylinder seal issue. The biggest pointer there is the pedal falling while your foot rests on it. Strong panic braking is also consistent of that because it's a sudden spike in pressure and a slightly damaged seal will still hold. Slowly falling pedal is a peak somewhere - oozing from the piston, leaking from a hose, or leaking past the master cylinder seal. While technically it could be an air or fluid "leak" past whatever seal it is, air isn't going to suddenly compress a little easier while you're holding your foot on the pedal, and a big bubble or air would give you panic stop issues too.

Something I don't feel I see mentioned enough - as cars age the master cylinder bore gets polished in the range where it is most heavily used. This makes it smooth and slightly larger diameter. Past that point the diameter goes back to original and there could be surface imperfections. Grind the seal past that, like during bleeding or some weird brake failure that causes the pedal to fall dramatically, you can now get a little tear in the master cylinder piston seal, for which there is no fix but replacement.

I see lots of bleeding instructions, including BMW's own, which say "pump the pedal to the floor". I specifically place a brick or block of wood under the pedal to keep it in normal operating range on my vehicles.

The car had it's original brakes front and rear when we purchased it January 21 with ~98k on the clock up until I changed the rears 6 months ago and the fronts yesterday. I assume that goes for the brake fluid also.

I have no reason to believe the pedal has ever been pushed lower than the 'usual'. However, I can't say for sure.

There's no leaks, hoses are good etc. This has to be the master cylinder?

I think it would be wise to do another bleed of the brakes. I think it's dry tomorrow night so I will pull another litre of fluid trough and see what happens.

 

[Marcozi]

This is why we should all replace the brake fluid every 2 years. Also, do not pump the brakes when changing the fluid. What happens is the seals in the master cylinder can be damaged when the brake pedal goes to the floor. Use either a pressure or vacuum bleeder. I use the later.

 

[Bailes1992]

This is why we should all replace the brake fluid every 2 years. Also, do not pump the brakes when changing the fluid. What happens is the seals in the master cylinder can be damaged when the brake pedal goes to the floor. Use either a pressure or vacuum bleeder. I use the later.

I used a pressure bleeder. I have a vacuum bleeder but always found it more work than the pressure bleeder.

I find it handy for draining the fluid reservoir though.

 

[Bailes1992]

Just wanted to update this.

Replaced the master cylinder as it was my gut feeling it had failed or was failing. I was kind of lured into a false sense of positivity for my diagnosis while doing the job. I noticed the new master cylinder was very difficult to actuate, needing quite a lot of force while I bled it. The existing master cylinder was very easy in comparison to actuate.

Installed it and very little difference. Maybe a small difference but honestly, it would have been impossible to say for sure.

So I did some more research and managed to download a copy of Pyclip which is a Renault/Dacia diagnostics tool and re-bled the brakes with the ABS pump running. Had a constant small stream of bubbles out of each corner with a slug of dark black brake fluid.

Brakes fixed!

For all of 2 days, I took the car out last night and really anchored on to see how things were. There was a 'pop' from the rear LH brake drum and now there's a pull to the right under light braking. If you pull on the handbrake while moving the rear LH brake drum is making a grinding sound. So looking forward to finishing work today to investigate!

 

[Jetronic]

so the answer was given in post #2

 

[mrbreezeet1]

At any time during your recent adventures (e.g. when the rear linings delaminated) did the brake pedal ever travel significantly further than in normal use?

The first thing that comes to mind from your description is a master cylinder seal issue. The biggest pointer there is the pedal falling while your foot rests on it. Strong panic braking is also consistent of that because it's a sudden spike in pressure and a slightly damaged seal will still hold. Slowly falling pedal is a peak somewhere - oozing from the piston, leaking from a hose, or leaking past the master cylinder seal. While technically it could be an air or fluid "leak" past whatever seal it is, air isn't going to suddenly compress a little easier while you're holding your foot on the pedal, and a big bubble or air would give you panic stop issues too.

Something I don't feel I see mentioned enough - as cars age the master cylinder bore gets polished in the range where it is most heavily used. This makes it smooth and slightly larger diameter. Past that point the diameter goes back to original and there could be surface imperfections. Grind the seal past that, like during bleeding or some weird brake failure that causes the pedal to fall dramatically, you can now get a little tear in the master cylinder piston seal, for which there is no fix but replacement.

I see lots of bleeding instructions, including BMW's own, which say "pump the pedal to the floor". I specifically place a brick or block of wood under the pedal to keep it in normal operating range on my vehicles.

Right you never want to let your pedal go to the floor when you're bleeding them I tell whoever's working a brake pedal for me don't let it go all the way to the floor just about 3/4 of the way!

 

[vwmaniaman]

My Ford E450 motor home had a bad right front caliper. The shop tried to bleed it and it would not bleed. They got out the sun tester and hooked it up and the ABS was making lots of noise and then it bled and the brakes were normal. Predal went to the floor before they used the tester. I have never seen anything do like that before.

 

[WylieCoyote]

My Shadow did this after a rear brake job. The shoes need to be adjusted right up against the drums in order for the pedal to feel right. Any slack, and shoe travel is too long for the system, and I get a lot of pedal travel.