I changed front rotors and bled brakes. They were hard for a few days and went soft. No leak in fluid (MC reservoir is still full). No leak in fluid near any wheel. Could I need to bench bleed the master cylinder?
Thanks
cliff
The master cylinder may be leaking internally. However, if it went dry during the bleeding process, bench bleeding it may be in order.
Usually you have to bench bleed an MC. I’d start there.
Sit with the car off and pump the brake pedal. It should get really stiff after a few pumps. Hold your foot on it with firm pressure. If the pedal slowly sinks then you probably need a new master cylinder. If the pedal stays firm then you’re probably getting air into the system.
I worked on some brakes recently on a car with with rear drums. One of the wheel cylinders was seeping. There was no evidence of this outside of the drums, and it didn’t lose any noticeable amount of fluid but had sucked in plenty of air. The point is that you can easily have a little bit of a leak that doesn’t show itself obviously - but still gives you air in the system.
Thanks for tips. I tried hitting the pedal and it never built up pressure. Just went to the floor over and over. Suggestions? Bench bleed then look for replacement master cylinder?
Thanks
cliff
If the master cylinder never ran dry while you were doing the brakes/bleeding earlier, then I wouldn’t even go through the trouble of the bench bleed. Chances are we’re talking about a 12 yr old MC with who knows how many miles, and if it never ran dry then there’s no reason to suspect an air issue with it. I would probably just replace it.
I was thinking the same. The car’s got 224k miles on it. Will get a replacement. Thanks for the tip!
Replacing the MC with a new one at 224K can’t be argued with. But first I’d eliminate the possibility that the calipers are leaking, either the bleeder screws or the piston seals. If they are you should be able to see fluid leaking out, and the fluid level in the MC should go down.
The cause is puzzling though … here’s some other ideas, might be of help …
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Sometimes when I bleed the brakes, the pedal goes a bit soft after a few days. But not the way you describe it, just a little soft, and lower. I am able to fix this by just bleeding again. I think air gets trapped in some niche or another, or maybe gets dissolved in the brake fluid, and it takes a while for it to come out. Re-bleeding a few days later then removes the rest of it.
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I had one car, the brake system on that one, DIY bleeding was close to impossible to get good results. With that car I’d have to take the car to a mechanic and have them do a pressure bleed. That always worked.
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If I’m in no hurry, sometimes I’ll do a gravity bleed. It’s very effective at eliminating the air in the system. Simple in concept. More difficult in practice. I simply open one bleeder screw and let it slowly bleed by gravity. No pedal pushing required. After a while and enough fluid leaks out, I close it, and open the next one, etc etc. This can all take a while to do, and needs to be monitored or all the brake fluid will leak out and you have to start again, and has another big downside if you forget you are doing this and try to drive the car with the bleeder screw open! No brakes! Maybe just taking the car in for a pressure bleed is the safer alternative.
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When you bled the brakes after replacing the pads, did you do it the normal way DIY’ers do it, by having an assistant press on the pedal 'til it nearly hits the floor, while you open and close the bleeder screw in sync with the pedal press? That process can damage the seals in a MC, b/c the MC piston seals are experiencing parts of the MC they normally never experience, and those surfaces inside the MC may well have some build up on them that damages the seals. That could explain why the MC suddenly could go bad.
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When you installed the new pads, you had to push the caliper pistons apart right? So the new ones would fit. How did you do that? You may have accidentally got the caliper pistons cocked slightly in the process, so now they are now not properly.
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If you decide to install a new MC, the installation instructions will probably have the bench bleeding procedure enclosed in the box. It’s easy enough to do, just be sure to hold it in the vice the way your are instructed to do, and not just clamp the whole thing in a vice any which way. This is to keep the cylinder perfectly round inside. And use a topredo level or some such gadget to make sure the MC is indeed perfectly level during the bleeding process.
Brakes can indeed be a frustrating job for the DIY. Best of luck.
I looked up the brake bleeding procedure for your vehicl
Start with the RH rear, then the LH rear, RH front, and then LH front.
Tester
Here’s something new. When I bleed the brakes only the LH front tire (closest to MC) has air coming out. Bad wheel cylinder? Again, nothing leaking - only soft pedal.
Usually there are two chambers in the MC, one for the front brakes, and one for the rear. So it is hard to understand how there could be air only on the front driver’s side caliper. You’d think if there was air getting into the “front” MC chamber, there’d be air coming out the pass’r side front wheel cylinder too. So I can see your wonderment.
hmmm … maybe you just have air in that line and you need to bleed it some more. Or maybe the driver’s side caliper seal is just faulty somehow, it doesn’t allow brake fluid to leak out, but it allows air back in when you let your foot off the brake. A one-way valve effect at a caliper seal wouldn’t be an impossible thing to happen I suspect. I think if I had this problem, with the symptoms you describe, and I was sure the bleeding had been done correctly and enough, then I’d remove the driver’s side caliper for a look see at the seals, maybe just replace it, see if that fixes the problem. If that didn’t fix it, next thing I’d do is replace the MC probably.
Sometimes you will get air leaking past the threads of the bleeder when it’s closed. You can bleed until the cows come home and wonder why there is still air in the system.
Sorry for the delay in final response. Talked through the situation and consensus was the MC. Most likely loosened up gunk when I first bled. I put the new one on and immediately felt a difference. Even before bleeding, the brakes felt great. Then did a complete bleed and all is well.
Thanks for the suggestions everyone!
cliff