Master cylinder troubles

I've been tearing my hair out for a couple weeks now trying to get this fixed right. the master cylinder on the truck quit a while back and i replaced it with a rebuilt one from autozone, which worked for about a week, and then started sinking. i replaced it with another rebuilt, and that one started sinking as soon as i had it installed in the truck. then i gave up on the zone and bought a "new" one from NAPA, which seems to do fine until i drive for a bit and then it starts sinking. I'm guessing that it is getting warm and the cylinder is expanding and compromising the seals.

what am i overlooking? am i just this unlucky to have gotten 3 bad ones in a row? i ordered another "new" master cylinder from NAPA that i can tell is not new, because it has tokico cast into it and they ground off where "nissan" is cast into the oem ones. i have another "new" m/c from o'reillys that actually looks new. i'm almost to the point of sending it to a shop and letting them deal with the headache

Are you absolutely sure that it's still a master cylinder issue and not one somewhere else in the braking system? While I've had occasional issues with duff new or remanufactured parts, getting three duff master cylinders from two companies sounds rather unlikely.

If you've got drum brakes and no visible fluid leaks, I'd be tempted to check the wheel cylinders if one of them is leaking.

the level in the reservoir doesn't change, also why wouldn't the front circuit keep the cylinder from sinking, even if the rears were completely gone

Hmm. Does it sink with the engine off as well or only when driving and/or with the engine running?

both. earlier today when i drove it, i had a firm pedal for the first 20-30 minutes and then it started sinking again. does it engine on or off once it starts.

I'm guessing you've completely bled the system and adjusted the drum brakes?

yes, three times now

That's a weird one. I can only think of three things off the top of my head that would cause the pedal to sink without losing fluid:

1. An internal leak. The only time I've personally had a sinking pedal where no fluid was lost was when the MC was bad. However, I can't imagine this problem occurring 3 times with 2 different vendors. Doesn't seem likely.

2. Fluid boiling. Is a line routed near the exhaust?

3. A flex-line ballooning? In part due to heat?

this is the factory brake system, with stock exhaust system. fuel and brake hard lines run down the right frame rail, exhaust runs on the left side.

its possible that a hose is bulging, but that would just make the pedal spongy, not sink to the floor no?

Is it ABS? One of my Corrados has a dying ABS pump, and it does that when the pressure's low. After warming up, the pump does a little better and my pedal returns.

Strizzo wrote: its possible that a hose is bulging, but that would just make the pedal spongy, not sink to the floor no?

Depends on how big the bulge is. Does it seem likely? No, but I'm running out of ideas. The symptoms sound exactly like a bad MC, But I've rebuilt so many master cylinders myself and replaced so many with "factory rebuilt" or new without ever having an issue, I just cannot fathom 3 in a row being bad...but it certainly is possible.

Remembering now another instance where something similar happened to me was when I tried to use silicone brake fluid in a Triumph Spitfire in the clutch hydraulics. Worked a couple of days, then it didn't. Changed back to Castrol. No more problems. But, I'm assuming you are still using stock brake fluid.

the truck has abs, but its rear only, and there is no pump, it just has an isolation valve, and dump valve, which opens the rear circuit to a plunger with a spring behind. i have a suspicion that it could be leaking past the dump valve under normal operation, and the sinking of the brake pedal is caused by that, but that doesn't explain why the front brakes won't hold pressure. the front circuit has no abs and the lines run straight from the m/c to the front calipers.

In reply to Strizzo:

Rear ABS...yes, that would bring in a few more variables. I think you might be on the right track. I'm not sure I understand the question about the fronts holding pressure, though. If the rears are losing pressure for whatever reason, the pedal is still going to sink even if the fronts are ok.

but why would the fronts not hold if the front circuit wasn't losing pressure?

one way that i checked to see if it was losing pressure was to stop on an incline and hold the brake pedal in one spot. then as the pressure bled off, the brakes would slowly release. yesterday when i started the truck, it wouldn't lose pressure, and i could press and release the pedal and it wouldn't drop. after driving for a bit, the pedal started falling, and stopping on the same incline the pedal would drop and/or the brakes would let go.

i would think that if the abs dump valve was leaking, it would be doing it all the time. also the m/c is exposed to more heat than the abs module.

so i've found what might be causing so many bad master cylinders. in looking at car-part.com to see what master cylinders run, i found that apparently at some point reg cab 4x4 trucks came with a 15/16ths master that looks (from pics on c-p.com) identical to the 1" master that came in all 4x4 trucks in later years (i think). if the 1" cylinders were rebuilt with 15/16ths seals this would explain why they all are sucking.

Strizzo wrote: so i've found what might be causing so many bad master cylinders. in looking at car-part.com to see what master cylinders run, i found that apparently at some point reg cab 4x4 trucks came with a 15/16ths master that looks (from pics on c-p.com) identical to the 1" master that came in all 4x4 trucks in later years (i think). if the 1" cylinders were rebuilt with 15/16ths seals this would explain why they all are sucking.

Yeah, I could see something like that happening. The rebuilder somehow ended up with the small seals without realizing it was a different part. They're gonna be furious when they find out..that's going to cost them a bunch of warranty money.

friedgreencorrado wrote:

Strizzo wrote: so i've found what might be causing so many bad master cylinders. in looking at car-part.com to see what master cylinders run, i found that apparently at some point reg cab 4x4 trucks came with a 15/16ths master that looks (from pics on c-p.com) identical to the 1" master that came in all 4x4 trucks in later years (i think). if the 1" cylinders were rebuilt with 15/16ths seals this would explain why they all are sucking.

Yeah, I could see something like that happening. The rebuilder somehow ended up with the small seals without realizing it was a different part. They're gonna be furious when they find out..that's going to cost them a bunch of warranty money.

not as furious as the guy that had to go through four (and counting) of them to find a good one.

My brothers GMC with rear ABS had a faulty ABS accumulator that led to similar symptoms.

did it trip the abs light? the abs system runs its check, and the light goes off. i'll try unplugging the abs module and see if the problem persists. i was looking into bypassing the thing, but haven't found any metric flare fittings or metric patch pipe.

Strizzo wrote: not as furious as the guy that had to go through four (and counting) of them to find a good one.

As the young'uns say these days, "Word up, bro." I'd be pretty smoked, too.

Also, if either of the circuits suffer a pressure loss, you'd see it in the master cylinder (and therefore, your pedal), since most modern designs split the circuits left/right (RF/LR and LF/RR) instead of front/rear for "safety" concerns.

In reply to friedgreencorrado:

this one has the rear(on the cylinder) circuit to the front brakes, forward circuit to the rears by way of the abs module. there are two ports for the front brakes on the same circuit. the frontiers use the same m/c, but with only one boss on the rear (on the cylinder) circuit drilled/tapped for fittings.

if i end up bypassing the abs and still have the same problem, i might try to get one of the later frontier m/c's and plumb in a T fitting from the one hole. maybe i'll have a little better luck going at it that way. hopefully it won't come down to that though.

heres a pic for reference. the two ports right next to each other go to the front brakes. on the later model frontiers, the port that runs in diagonal isn't drilled. the port towards the front of the cylinder out by itself runs to the rears.

so, and update.

i replaced the master cylinder (this makes number 4) with the new one i ordered from oreilly. this one had "USA" cast into the body, as well as a serial number stamped into it. first impressions are good, i have a firm pedal and drove around for the better part of an hour last night seeing if it would start dropping. good so far, but it was cool out so im sure the underhood temps didnt' get to where they had been before. i'm keeping fingers crossed for now though