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I have a 2003 Chevy Malibu with disk breaks on the front and drums on the rear. Girlfriends complaint was that they squeak all the time when used. I looked them over and decided to replace the rotors and pads up front and the drum shoes and drums on the rear. They were pretty darn rusty. After replacing everything I ended up with a squishy brake peddle. I could push it almost to the floor. However, There is enough breaking going on to drive the car but I do not have enough brake power to “lock” them up. The car does not have ABS.
So, I decided I would bleed the brakes. I could not get the bleeder screws lose on the front disks and decided to try and bleed them from the brake line.
After that I checked the back cylinders on the wheels and also could not get the bleeder screw lose. So, I replaced the cylinders also on the back wheels and bled the back brakes.
I still have the soft peddle so I made the decision to replace the front 2 calipers so that I could actually bleed the front brakes. I didnt feel very confident trying from the break lines. So, I replaced the front calipers and bled them.
Still a soft peddle.
I did push the front calipers in about 1/8 th of an inch to get them over the front brake shoes. Could that have been enough to damage the master cylinder somehow?
Perhaps I got air into the master cylinder somehow, Could that be my problem?When bleeding the brakes the level in the master cylinder did go down and I do not see any leaks anywhere from my work.
Any opinions on this? I wonder what I screwed up. If I accidentally got air into the Master Cylinder how would I get it out?
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Yes, pressing the caliper piston in could have done it. Always open the bleeder (or crack the brake line) when compressing the piston. The MC isn’t designed to accommodate that much pressure in the reverse direction. It’s designed to prevent backflow. Otherwise you would have to pump the pedal every time you want to stop. Forcing the piston against the normal flow violates this design and can blow the seals. Sometimes there are rebuild kits available (I couldn’t find one in my Dorman catalog for your car), but I’ve always found it better to just swap them. I’ve rebuilt a few, and it isn’t terribly difficult, but can be messy, and something I’d rather not deal with most of the time.
Before condemning the master, one last thing to check would be the rear shoe adjustment. With the parking brake released and the wheel off the ground, it should only rotate 1/4 to 1/2 turn when spun by hand. If it travels more than this, that can cause a spongy pedal as well. If the rears are ok, then the MC is most likely the issue. EDIT: I see you posted that you already checked the rears.
FWIW, I just looked on RockAuto and they have aftermarket reman’d for $50-60, and new OEM for around $145. The ‘zone in my area lists reman’d for about $75.
[quote=”Evil-i” post=139733]I seriously doubt that pressing in the caliper piston would have damaged the master cylinder. It’s part of the job when installing new pads.[/quote]
No, it’s only part of the job once you crack the bleeder or brake line first, as I explained in my previous post. Those are one-direction circuits in the master.C-clamp used to push it back in.
I am investigating the possibility I somehow damaged the Master Cylinder. (??)
Lots of googling going on.
Hey Eric, How about a video on troubleshooting master cylinders and brake boosters ?
Thanks,
[quote=”cap269″ post=139735][quote=”Evil-i” post=139733]I seriously doubt that pressing in the caliper piston would have damaged the master cylinder. It’s part of the job when installing new pads.[/quote]
No, it’s only part of the job once you crack the bleeder or brake line first, as I explained in my previous post. Those are one-direction circuits in the master.[/quote]I assumed the bleed valve was open when OP did this. If he’s mentioned that he did it with the valve closed, I missed it.
[quote=”Evil-i” post=139737]I assumed the bleed valve was open when OP did this. If he’s mentioned that he did it with the valve closed, I missed it.[/quote]
The OP wrote:
After replacing everything I ended up with a squishy brake peddle. I could push it almost to the floor.
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So, I decided I would bleed the brakes. I could not get the bleeder screws lose on the front disks and decided to try and bleed them from the brake line.That told me he didn’t open the bleeders or crack the line when he changed the pads.
My posts are going out of sequence a bit. Sorry.
Ok, I have checked the drums in back. I fear that I damaged the MC. I did not crack the bleeders when I pushed them in. Therefore, It is looking bad for me.
I do not normally screw up. Its just on occation that I do these things.
I am very embarrassed if this is the fact.
coffee
That is correct. I screwed up.
To better understand MC’s what actually happens inside the MC ? I have been looking at diagrams of your basic MC’s and studying them.
Ok, a crash course in dual master cylinders. A long time ago brake master cylinders had only one piston and reservoir, and one hydraulic outlet. This outlet fed a manifold that distributed fluid pressure to each of the four wheels. This worked well. Until there was a leak and all four wheels lost pressure. To combat that problem and improve safety, the dual master cylinder was designed. This had two chambers and two pistons, and two outlets, so that if there was a leak, only two wheels lost pressure and the other two could still stop the car. However, the mechanism became a bit more complicated. Modern dual cylinders have additional switches and devices, especially ABS-equipped vehicles, to improve safety and thus have become even more intricate. Below is a diagram of a dual master cylinder. Take note of parts 15 and 17 which are cup seals (see pic of cup seal if you are unfamiliar).
Cup seal:
When your foot pushes the brake pedal, it operates a pushrod that depresses the primary piston. Fluid pressure then builds between the primary and secondary piston so that the secondary is activated. Here is a short video that illustrates how these pistons work together.
https://www.youtube.com/watch?v=RAcRMqCSlCs
Now that you have seen the diagram and watched the video, let’s discuss what happens when fluid is pushed backward. Normally, when you press the brake pedal, there is forward travel on the pushrod and pistons. When you release the brake, the fluid pressure that accumulated in the lines is able to relieve and this pushes the pistons back to their original home positions within the master cylinder. Newton’s Third Law of mechanical physics states that every action has an equal and opposite reaction. You push the pedal, the pedal pushes a pushrod, the pushrod pushes a piston, the piston creates pressure, the pressure pushes the next piston, the next piston creates pressure, those pressures are sent to their outlet circuits, which activates the caliper piston or wheel cylinder at the other end. That is the action. The reaction is, when the activating force at the beginning (your foot) is removed, the process reverses, and the pistons are returned to their home position. When you force the calipers pistons back into their bore with a clamp or other device, the MC pistons are already in their home position because the pushrod hasn’t been activated. Now, there are many foot pounds of fluid force being applied against the pistons and they can’t move because they have no where to go. This force can’t just dissipate into thin air, because that would violate Newton’s Law. Instead, the action of compressing the caliper piston causes the reaction where the fluid force is converted to work, and that work, in this case, is destruction of the seals in the master cylinder (especially the one marked #17 in the diagram). Once the seal has ruptured, the operating mechanism of the master cylinder is affected because the seal can no longer sustain the fluid pressure building up to properly move the MC pistons. The fluid pressure escapes through the rupture, and flows into the compensating circuits (because the pressure was not sufficient to move the pistons enough to block off the compensators) where it is equalized and expended to the outlets. This is commonly referred to as ‘bypassing’ and causes the spongy pedal.
I think I have the problem figured out.
Did some studying of MC’s and noticed that when I bled the MC in the car it was not level. I have seen info that in order to bleed the MC correctly take a torpedo level and put it on top of the MC. Where the air bubble is shows where the trapped air in the MC will be. Air bubble in Torpedo level must be towards the front seat. Then when pushing down slowly on the brake it can push the air out the ‘equalizer hole (s).
That may very well be my problem. I do not find any air in my lines. : )
Girlfriend has the car and is away right now for two weeks so it will be a while before I can look at it again.
Thanks for all the helpful replies.
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