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I’ve just replaced the brake booster and master cylinder in my 1993 Jeep Grand Cherokee 4.0L. The booster was leaking and I figured I might as well give it a new MC as well.
Did a 50-mile drive today, first proper one since doing the work. The brake light on the dash came on several times during braking, but each time it went off again when I pumped the pedal. Actual brake performance seems normal.
Fluid level is fine, no obvious leaks anywhere. I bench-bled the MC before installing, and I also did a full factory bleed – complete manual bleed of MC, combination valve, ABS input lines, and wheels, then a DRB bleed of the ABS and then another full manual bleed.
I did wonder if it was just the pressure switch, since I broke one of the two little plastic tangs that secure its harness connector to the valve when I disconnected it to get some working room – but if it was just an intermittent electrical connection then presumably pumping the brakes wouldn’t turn the light off, and it does.
Something else I’ve noticed is that the pedal slowly sinks while I’m holding the Jeep on the brake at lights, but as I say, actual brake performance seems normal.
Any ideas? I’m obviously hoping that I didn’t get a faulty MC (it’s a Cardone reman) since all that bleeding is such a tedious, messy PITA…
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I’d start with obvious things first, fix that electrical connection and then check the mc and booster. How are the brakes looking wear wise?
My instinct is that it’s not an electrical connection issue. If it was, then pumping the brakes wouldn’t make the light go out again, surely?
Just to reiterate, the actual connector to the pressure differential switch is fine, it’s just one of the two plastic tangs on the connector that broke when I removed it.
I’d bleed the system again, it’s a mess but getting air out of these systems is always a pain. That’s the only thing I can think that would cause the pedal to “sink”.
OK, just drove it home and concentrated on how the brakes were working. With typical short applications of the brakes, such as adjusting speed in traffic, the light stays off. If I press and hold the pedal, it will slowly sink until the light comes on. When that happens, I need to pump the pedal a couple of times to turn the light off. This is a consistent thing. There’s definitely no fluid leaking.
Given the above confirmed symptoms, could that still be as simple as air in the lines rather than my new master cylinder being faulty, or a bad proportioning valve?
There are 2 things that often turn on that light. Low brake fluid, or the parking brake switch. Given that you just replaced the master cylinder, perhaps there’s an issue with the level switch. I also suggest you address the known issue with the electrical connector you mentioned.
As for the pedal sinking to the floor. That’s actually a symptom of a bad master cylinder. Not all parts are created equal. Perhaps the part you purchased is the issue here.
Good luck and keep us posted.
[quote=”EricTheCarGuy” post=115651]There are 2 things that often turn on that light. Low brake fluid, or the parking brake switch. Given that you just replaced the master cylinder, perhaps there’s an issue with the level switch. I also suggest you address the known issue with the electrical connector you mentioned.
As for the pedal sinking to the floor. That’s actually a symptom of a bad master cylinder. Not all parts are created equal. Perhaps the part you purchased is the issue here.
Good luck and keep us posted.[/quote] I agree with Eric as I have experienced this myself. Aftermarket mass produced off the shelf master cylinders are dare I say “inferior” and not created equal. If any of you recall I was having master cylinder problems as well and it turned out to be the fact the aftermarket cylinders did not have the correct bore size as well as not being able to handle the leverage of the stock assembly(hydroboost parts combined with vacuum parts nightmare!). With an OEM reman the problem went away. My case was somewhat unique but the same principle applies.
[quote=”EricTheCarGuy” post=115651]There are 2 things that often turn on that light. Low brake fluid, or the parking brake switch. Given that you just replaced the master cylinder, perhaps there’s an issue with the level switch[/quote]
There’s no level sensor on mine. The only electrical connector is the pressure differential switch, which is part of the proportioning valve assembly.I don’t believe that snapping off one of the plastic tangs on the connector housing has caused this problem – I have other connectors in the engine bay that have near-enough fallen apart where the plastic has become brittle and snapped, and they all still work 🙂
The light comes on when the pedal sinks all the way to the bottom of its travel, and it goes off when I pump the pedal. This suggests that the pressure switch is working correctly – it’s detecting a pressure imbalance between the two hydraulic circuits when the pedal has sunk all the way, and it’s also detecting that the pressure has been re-equalised when the pedal is pumped.
The MC is a remanned OEM unit; AFAIK Mopar discontinued the MCs for the Jeep ZJ years ago. The consensus elsewhere is that reman MCs are notorious for a high rate of out-of-the-box failure (something I didn’t know before). A few manufacturers sell brand-new ones, so I need to get some recommendations for brands.
It could be you just got a dud out of the box.
They carry a warranty. If it was mine? I’d replace it under the warranty “ONCE”. If the replacement one also has issues? Then I would ask for a refund and chase down a different reman company–or even go with a new one or a factory OEM rebuild.
And yeah, bleeding brakes can be a messy job. That’s why I love using the vacuum brake bleeder setup.
S-
So the fact that the pedal is slowly sinking is more likely to indicate actual MC failure than simply trapped air?
Normally air in the system makes for a brake pedal that feels like you’re stepping on a sponge. Very mushy.
Part of the safety inspection we used to do (it’s been a few years sense I’ve been a licensed inspector) We would pump up the brake pedal and it had to hold for one minute without sinking. If it made it that minute without sinking we then start the car and expect the pedal to fall a bit for power brake booster check.
If the pedal slowly sinks, and there is no sign of any leakage? Even money bet the master cylinder is having one of the cups leaking.
S-
+1 Normally trapped air gives you a spongy feel. If the pedal sinks to the floor that normally indicates a leak. In your case if you don’t have any other leaks the most likely cause is the master cylinder.
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