Brake problems after changing pads

[Entropy]

I changed the pads on my MX5 yesterday as part of regular maintainence. Everything was working fine. I changed the pads following Haynes instructions and thought all was OK as I had firm brake pressure. Took it for a test drive to find that the pedal goes to the floor.

I cannot find any leaks and didn't bleed the system when I changed the pads. So now when the engine is off the brake pressure is good but as soon as I start the engine the pedal goes to the floor.

Any ideas?

 

[propersplitbrainme]

Just changing the pads obviously shouldn't involve breaking into the system so it shouldn't need bleeding as you say. I assume you had to push the pistons back into the calipers in order to fit the new pads, have you pumped them back out again to take up any slack? Did you kink the brake lines when you did the job? Is the fluid level OK?
Other than that, try bleeding them.

 

[Entropy]

Just changing the pads obviously shouldn't involve breaking into the system so it shouldn't need bleeding as you say. I assume you had to push the pistons back into the calipers in order to fit the new pads, have you pumped them back out again to take up any slack? Did you kink the brake lines when you did the job? Is the fluid level OK?
Other than that, try bleeding them.

Already done all that. Brakes are firm when engine is off and nothing there when engine is running.

 

[needsabiggerfuse]

Brake pressure is there when the engine isn't running, but as soon as you've got servo assistance ... . You might have got one of your pistons stuck in the caliper as you pressed it back in. Brake Cleaner spray ... invaluable.

 

[Gord]

Brake pressure is there when the engine isn't running, but as soon as you've got servo assistance ... . You might have got one of your pistons stuck in the caliper as you pressed it back in. Brake Cleaner spray ... invaluable.

Surely if the piston get's stuck then the new pads would not fit, unless the piston was all the way in before becoming stuck. You should have some brake even so as the other brakes would still be activated.

You may need to bleed the brakes several times and ensure that your master cylinder is topped up to max before and after bleeding. You may have bled the brakes to the point where the fluid level in the master cylinder became so low that you sucked air into the system.

Truthfully, if you don't open the system, there should be no need to bleed the brakes as I'm assuming the master cylinder had fluid in it before you started the job so as you pushed the pistons back in, the fluid in the lines would have been forced back into your master cylinder. In fact I have seen it where the master cylinder has overflowed once the pistons have been pushed back in due to the addition of fluid before you got to the point where you decided to change the brake pads.

If all the pads are in their proper place both inside and outside pads on all four wheels, remove the wheels while supporting the car on jacks or axle stands and have someone start the engine and slowly operate the brakes so that you can see if there is any movement of the pads. If not, and there are no leaks and no air in the system then it could be an internal leak in your master cylinder that may have prompted you into changing your pads in the first place thinking they were worn down to the point of needing to be replaced..

A bit of a pain to remove and replace but not really all that difficult to do with the right tools. If that is the problem I would suggest going for a rebuilt master cylinder rather than buying the kit to rebuild it yourself as the bore may need to be honed and that can be a pain in the arse, including that you have to ensure that you don't leave any sh!te inside the cylinder bore which will cause it to leak again as it damages the seal.

It's really hard to say what could be wrong other than what's already been suggested without actually being there and seeing things in action.

 

[needsabiggerfuse]

Are your discs scored? If so, the extra ooomph given to your right foot by the brake servo when the engine is running, will be trying to push the flat faces of the new pads into the grooves.

 

[Gord]

By the way, brakes always seem to have more pressure on the pedal when the engine is not running due to the fact that you are not getting any boost assist so it could be that although you THINK the brakes work better with the engine off, they don't, it's just you having to do the work that is normally done by the booster.

Bleed your brakes, ALL FOUR OF THEM several times by having a second party pump up the brake pedal perhaps half a dozen times and hold the pedal to the floor as you open the bleed screw and KEEP IT THERE until you re-tighten the screw before pumping again DO NOT ALLOW THE PEDAL TO MOVE UP WHILE YOU STILL HAVE THE BLEED SCREW OPEN as this will suck air into the system. You'll need to do this several times with each wheel until the pressure comes back to ensure that there is no air in the lines and as I said MAKE SURE the master cylinder is full before doing this and check intermittently that it is still full after you bleed each wheel and top up as necessary.

Start with the wheel furthest away from the master cylinder, on UK cars that would be the left rear, then do the right rear, then the left front and finally right front. but don't think that simply because you have opened the bleed screw once and got some air out that you've done the job, usually you have to repeat the procedure several times on each wheel in order to accomplish this, always keeping an eye on your fluid level in the master cylinder.

It's always wise even when you think you've eliminated all the air in the system to go round each wheel and bleed them one more time as above just to make sure. You may think you are wasting a lot of brake fluid but brake fluid is a lot cheaper than body work or personal injury or an insurance claim against you.

 

[Entropy]

Thanks guys, I took today off work and had another look. I stripped the brakes down to ensure that the pads were fitted correctly and also pumped the pistons out slightly to see if they were stuck. I started to try to bleed the system but the nipple on the first brake was fairly solid so I decided not to continue. All was OK and I reassembled, the test drive was still the same.

As I need the car I admitted defeat and called the MX5 garage, they were quiet so took it down there this afternoon. They confirmed that there were no leaks but did say that the feed into the brake system is above the min line on the reservoir (????) so it is probably air in the system. Also they measured the disks and said they were really worn. When they held them up against a good set (currently off another car) I could see how bad they were. So its in the garage overnight and should get it back tomorrow.

 

[Gord]

Thanks guys, I took today off work and had another look. I stripped the brakes down to ensure that the pads were fitted correctly and also pumped the pistons out slightly to see if they were stuck. I started to try to bleed the system but the nipple on the first brake was fairly solid so I decided not to continue. All was OK and I reassembled, the test drive was still the same.

As I need the car I admitted defeat and called the MX5 garage, they were quiet so took it down there this afternoon. They confirmed that there were no leaks but did say that the feed into the brake system is above the min line on the reservoir (????) so it is probably air in the system. Also they measured the disks and said they were really worn. When they held them up against a good set (currently off another car) I could see how bad they were. So its in the garage overnight and should get it back tomorrow.

Probably the best plan mate, hopefully they will put new bleed screws in the calipers and put the little rubber caps on them to protect them against corrosion. If they don't put caps on, I would suggest that you slap a dollop of grease over the bleed screws to accomplish the same, this will make it easier to loosen the bleed screws the next time you have to do a brake job, if needed, but remember what I said about it usually not being necessary to bleed them AS LONG AS YOU DON'T OPEN THE SYSTEM as the fluid will be pushed back into the reservoir when you push the pistons back in and there should be no air in the system at that point. Check the fluid level in your master cylinder reservoir once a month or whenever you check your engine oil to make sure it's not gone so low that air can enter the lines.

ps You always have to operate the brakes a few times after a brake job in order to seat the pads against the rotors or it will feel as though you have no brakes the first few times you try to stop. They will do that at the garage.

Just a tip my local mechanic gave me, when replacing brakes, always get cheap rotors (disks) but the best pads as just about all rotors are throw away now, they don't turn them down as they are generally not made thick enough to do so anymore and even if they are able to be turned down it will cost almost as much to get them turned as it will for new rotors. I generally replace my rotors whenever I have to replace the pads or perhaps every second pad change depending on the condition of the rotors but over here I can buy new rotors for about $150 a set of four and the best brake pads cost around $80 for the front' and $50 for the rears, although I must admit I get a good discount because the company I worked for does a lot of dealing with NAPA, a major parts supplier in Canada and as a retiree I am entitled to this discount so I get them cheaper than the above quotes.

 

[needsabiggerfuse]

New disks, pads, and a freshly bled system it is then! Make sure you're wearing your seatbelt when you jam on your brakes for the first time or else you'll bend your nose on the inside of your windscreen. DT_Xtremez_42:

 

[Entropy]

I'm still half expecting a replacement master cylinder. What has really annoyed me is that I have replaced the pads on this car at least half a dozen times and I now have a problem. This is the first problem that I have had with the car since I got it back on the road last Feb and I've 15K already. Not bad for a 24yr old.

 

[Gord]

I'm still half expecting a replacement master cylinder. What has really annoyed me is that I have replaced the pads on this car at least half a dozen times and I now have a problem. This is the first problem that I have had with the car since I got it back on the road last Feb and I've 15K already. Not bad for a 24yr old.

A 24 yr old person or 24 yr old car:biggrin::biggrin:

I've got a '97 Mazda 626 2.5ltr V6 engine with almost 500K kms on it, that's about 300K miles, original engine and manual tranny, original clutch due to the fact that it was used for mostly highway driving 100+ miles each way from home to work and back. Gone through shedloads of rotors, brake pads and several sets of struts. Oh yeah and three front ends due to hitting stupid deer that decided to commit suicide by jumping out of the bush in front of me. Had to have a helicoil installed in number 3 spark plug hole due to the plug deciding to evacuate the area and stripping the threads. Other than that it's been a great car. I recently upgraded to a 2000 version of the same car as I need another new front end on the '97. I think I'm a dedicated Mazda driver from now on, well I have been for some time truthfully, as this one has 240K kms on it and runs like a dream.

 

[needsabiggerfuse]

I'm still half expecting a replacement master cylinder. What has really annoyed me is that I have replaced the pads on this car at least half a dozen times and I now have a problem. This is the first problem that I have had with the car since I got it back on the road last Feb and I've 15K already. Not bad for a 24yr old.

Half a dozen sets of pads and I presume you're still on the original disks. Well they don't owe you anything do they! You will notice a difference once the new stuff is bedded in. If you do actually need a master cylinder, make sure it's a branded new 'un; recons and 'knock-offs', noooooooo.

 

[Gord]

Half a dozen sets of pads and I presume you're still on the original disks. Well they don't owe you anything do they! You will notice a difference once the new stuff is bedded in. If you do actually need a master cylinder, make sure it's a branded new 'un; recons and 'knock-offs', noooooooo.

Re-built ones are ok if they've been done by a recognised supplier as they send them back to the factory. You don't want one from Bob's garage down the street and around the corner somewhere