Problems after replacing master cylinder

Hi,

I have a lexus es300 with 128K miles. I had taken my car to the mechanic as I felt the brakes needed work. I had to press the brake pedal all the way down to stop the car. The mechanic suggested replacing the master cylinder which I did.

Now, I have 2 unique problems and I am looking for advice here to educate myself before I take the car back to the mechanic.

  1. The brake pedal sometimes works just by a little pressing, and other times I need to press it all the way down. In other words, sometimes it brakes just by a slight touch of the foot and sometimes it doesn't. The brakes work, just that the "firmness" of the pedal keeps changing.

  1. The car starts vibrating after driving for about 10 minutes on the highway. The vibration doesn't start right away. There is vibration in the car and the steering. When I brake, I get the pulsating feeling with the brake pedal. All the passengers in the car can feel the vibration. After about 15 minutes of driving, the vibration goes away and doesn't come back during the same ride. If I park the car, let it cool and drive again, then the same pattern repeats.

Could the experts here give some insight into what the possible issues could be? The advice here has always helped me in the past and look forward to your opinion.

Thanks and regards, Mark

Reply to
Mark
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sounds like air in the system. needs to be bled by someone that knows what they're doing - your previous "mechanic" clearly doesn't.

remove wheels, scrape all rust from the wheel mounting surfaces on brake disk. apply a LITTLE antiseize, making sure you get none on the disk, then re-fit the wheel with bolts being torqued with a proper torque wrench, in a two-stage sequence, and in the proper order.

Reply to
jim beam

Mark wrote in news:e52c66b9-90a0-4583-a493- snipped-for-privacy@w17g2000yqj.googlegroups.com:

What year?

Does it have ABS?

Reply to
Tegger

Sorry, should have mentioned in my post. It is a '97 es300, and yes it does ABS.

Reply to
Mark

I would suspect a bad vac booster - assuming it has normal vac boosted brakes and not boosted by power steering pump.

Could be tire(s) flat spotting when they sit. Could also be loose / worn suspension or steering parts.

Reply to
Paul

Your problem sounds like the booster is not releasing which results in your brakes being partially applied. This explains both the low inconsistant pedal and the brake pulsation after driving a couple of minutes. You may be able to use your toe and lift the pedal to clear the problem until you apply the brake again. If the master cylinder was replaced, the mechanic may not have been able to effectively bleed the system to purge air from it.

As an aside, it is usually best to replace both the booster and master cylinder if one fails and they are essentially the same age with significant miles on them. In you rcase at this point, I would have the booster replaced and the hydraulic system purge following the procedure in the manufacturer's service manual to be sure the ABS system is cleared. That year model may also require a scanner to correctly purge the ABS controller. You may want to have the brake pads checked for heat damage, glazing, cracking, disintegration, etc. to make sure the brakes are good to go. I had this happen on my CV a couple of years back right after a brake job. The pads were destroyed in less than 50 miles after the problem began.

Lugnut

Reply to
lugnut

This part sounds like a stuck caliper. What happens is when you apply the brakes, the pressure is being applied to only one side of the rotor because the caliper can't move freely. That means even the tiniest amount of run out in the rotor causes pulsation. If the caliper weren't stuck it would float with the sideways movement of the rotor. If the caliper can move freely a small amount of runout wouldn't cause pulsation. It also sounds like when things get warmed up the problem disappears. That would mean the problem is related to temperature. If your description is accurate as things start to warm up the problem gets worse before it gets better. This is unusual and I can't think of a reason that would be happening, but possibly something wasn't assembled correctly.

if you have a stuck caliper things could be getting extremely hot. That could explain the intermittent pedal to the floor if you are getting things hot enough to boil brake fluid.

-jim

Reply to
jim

Mark wrote in news: snipped-for-privacy@w17g2000yqj.googlegroups.com:

Why that as opposed to the pads or bleeding the air from the lines?

Reply to
chuckcar

Mark wrote in news:f4392a29-ced5-4ae9-80c7- snipped-for-privacy@s9g2000yqa.googlegroups.com:

Do you have any freeplay at the pedal?

Push the pedal with your fingetips, as lightly and gently as possible. You should feel the tiniest bit of very easy movement before it hits something solid and effort becomes much greater. You're looking for something like this in the first 1/16" or 1/8" or so of pedal travel.

Reply to
Tegger

Thanks everyone for your replies. I'll take the car back to the mechanic and have it checked again. I'll also have him look into the suggestions made here regarding proper bleeding, checking the vac booster, the caliper and rotors. The pads and rotor were fine at the time of replacing the master cylinder.

As a side note, I had the tires rotated and wheels balanced a couple of times, but that didn't fix the problem. I have also been getting the check engine light with code P0446 (Evaporative Emission Control System Vent Control Circuit Malfunction). I didn't think it could be related to the problems I am having, but wanted to mention it, just in case there is a possibility of it being related.

Tegger: I will check for the freeplay in the brake pedal and report back.

Thanks for your help, Mark

Reply to
Mark

What's probably happened is that the rod which is connected to the booster was not adjusted to the new master causing the brakes to stay slightly applied. We live in hilly area here in BC Canada and run into a problem called warped rotors. As the rotor gets overheated from over braking, the two parallel surfaces of the brake rotor becomes warped. These unparalleled surfaces cause the pads to move unevenly producing the shake in the steering wheel and pulsation in brake pedal. The heated rotors and pads cause a condition as brake fade. This codition requires more pedal effort as brakes get hotter. The shaking is reduced ss the rotors cool . The rod that pushes on the masters' piston is adjustable for length to set free play. Too much length and the master will keep pressure to the braking system. Setting proper lenghth will solve this problem. The only other problem is to fix the warp rotors you need to machine or replace the rotors back to two parallel surfaces.

Reply to
<address_is

Air leak (intermittent) between the booster and intake manifold. You lose vaccum and the brakes need a lot more pedal pressure. When the leak occurs, the engine runs rough and shakes.

Reply to
Paul Hovnanian P.E.

"Paul Hovnanian P.E." wrote in news: snipped-for-privacy@Hovnanian.com:

If it were an air leak, the idle would be elevated.

Rough-idle and shaking were characteristics of non-feedback systems that were unaware of loss of manifold vacuum. OP's Lexus has a feedback system.

Reply to
Tegger

Not if the ECU compensates for the leak.

But a feedback system would compensate for the average error. If the vacuum for the brake system is tapped off a part of te manifold nearer one cylinder, it will throw that one's mixture off.

Reply to
Paul Hovnanian P.E.

latitude to /decrease/ idle air is limited, and of this type, insufficient. once there is a leak, the ecu increases fuel in ratio to the air supply to maintain stoichiometry.

and increase fuel delivery...

>
Reply to
jim beam

"Paul Hovnanian P.E." wrote in news: snipped-for-privacy@Hovnanian.com:

The ECM/PCM compensates by correcting the mixture AND the idle speed, or it can compensate by correcting the mixture and ignoring the idle speed. OP's Lexus probably does the former. Both our cars do the latter.

In any case, a feedback-system the ECM/PCM would compensate for an air leak by shortening injector pulse width until fuel mixture and/or idle speed were correct again. There would be no rough-running or shaking.

Possibly. But considering that vehicles with MAP sensors typically have only one sensor for the entire intake manifold, I'd think pressure changes at one cylinder would have minimal effect overall; certainly not enough to induce roughness and shaking.

Reply to
Tegger

10 m "After about 15 minutes of driving, the vibration goes away and doesn't come back during the same ride."

If I read that correctly the vibrati "The brake pedal sometimes works just by a little pressing, and other times I need to press it all the way down."

If the booster isn't getting vacuum then the opposite would happen. The pedal would not go down as far as normal and the braking power would be reduced.

-jim

Reply to
jim

Tegger wrote in news:Xns9D5A56829A77Ftegger@208.90.168.18:

Sorry, should have read "LENGTHEN pulse width" (to compensate for the excess air).

Reply to
Tegger

Are you folks suggesting that the P0446 CEL and the problems I described earlier with the vibrating and brake master cylinder are related? Sorry, some of this is way too technical for me. Thanks !

Reply to
Mark

No I don't think anyone suggested your DTC code and the brake/vibration were related. The code means it failed one of the diagnostic tests that are done on the system designed to keep raw fuel vapors from getting into the environment.

-jim

Reply to
jim

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