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Become
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Joined: August 25, 2008
Posts: 8
Location: Duluth, GA
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 Posted: Wed Aug 04, 2021 4:27 pm Post subject: Help with a funky brake issue |
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Hoping you folks can save me from just throwing a bunch of parts at this weirdness.
I changed my rear shoes, soft lines, and cables yesterday which all went fine. Brakes work fine and stop good, but every once in a while the pedal just goes soft. One pump and they are back again. No leaks detected. System was bled twice and had clear fluid with no bubbles both times. One thing that keeps sticking in my head is that my buddy that worked the peddle while bleeding commented that he got pressure when we did the passenger front (third corner bled), but it would dissipate when holding. I also remember hearing more fluid movement at the corner working the bleeder.
Really the only things left that I can think of are the master, front soft lines, and calipers. Don't think it's the lines. The experience when bleeding the passenger front makes me think it could be the caliper, but I would think it would be more consistent if so. That flips me to the master going out.
Any ideas from the group? Or should I just drop the coin and replace all the things? |
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vwpieces
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Joined: March 23, 2005
Posts: 345
Location: UpHill BothWays, PA
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| Posted: Wed Aug 04, 2021 6:31 pm Post subject: Re: Help with a funky brake issue |
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You may have moved the seals inside the master through the bore where rust is.
Possible the master is shot, but you may be able to take it apart and clean it. But no guarantees that the seals aren't already damaged. Dirt in master may have also been stirred up and stuck inside a check valve. Some hard fast pressing of the pedal Might help dislodge dirt.
Pressure bleeding or vacuum bleeding helps avoid the master seals going into the bore where they haven't been for a Long time.
Always a possibility you let the reservoir get too low on fluid. any suspicion of that you should start over.
Another note: I would have replaced the front hoses too. Especially if they have not been changed in the past 15+ years.
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W1K1
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Joined: March 04, 2004
Posts: 4919
Location: Southern AB
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| Posted: Thu Aug 05, 2021 7:14 am Post subject: Re: Help with a funky brake issue |
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i had a sinking pedal once, it was a leaky aftermarket brake light switch, just a drip, and the pedal felt good pumped up, but then it would slowly start to fade if you kept pressure on it
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raygreenwood
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Joined: November 24, 2008
Posts: 21512
Location: Oklahoma City
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| Posted: Thu Aug 05, 2021 8:15 am Post subject: Re: Help with a funky brake issue |
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| vwpieces wrote: |
You may have moved the seals inside the master through the bore where rust is.
Possible the master is shot, but you may be able to take it apart and clean it. But no guarantees that the seals aren't already damaged. Dirt in master may have also been stirred up and stuck inside a check valve. Some hard fast pressing of the pedal Might help dislodge dirt.
Pressure bleeding or vacuum bleeding helps avoid the master seals going into the bore where they haven't been for a Long time.
Always a possibility you let the reservoir get too low on fluid. any suspicion of that you should start over.
Another note: I would have replaced the front hoses too. Especially if they have not been changed in the past 15+ years. |
While this is all 100% true......if you have rust anywhere in the MC within tbe stroke range of the piston seals.....the cylinder is shot and needs to be reworked (if its not too late with rust pitting too deep).
Hydraulic pressure on the seals is only a stroke limiter on very light braking. Otherwise.....both piston sets have stroke limiter tails built onto the pistons. If you have rust anywhere witin the max stroke lengths of either piston circuit......you need to disassemble and either lap the cylinder or replace it.
Treating the MC "limply" to stay out of potential rough areas is the wrong way to go because you cannot guarantee that in real life.
Also......there should never be ANY trash, grit or debris in the master cylinder. It either comes from being there from the start .....filthy when new.....which is very common....which is why I strip and rinse in denatured alcohol on every new cylinder.....or you have no strainer under the lid on your reservoir or you are not using an outer boot.....all are a no-no.
But the effect you are describing is spot on with any of these issues:
A. What you get from NOS MC seals that have hardened and taken a shape set in the bore over time. They cannot stay inflated without heavy pedal pressure.
B. Very old seals in a non-NOS cylinder
C. Rust pits in the cylinder wall down in the bore...which actually takes a LOT of water in the brake fluid over a long period of time.
D. Rust ring around the entrance end inside the boot....caused by no boot or a leaky boot and almost always combined with leaking from the outer end piston seal. Running over this rust repeatedly grinds up the outer seal.
E. As noted.....a piece of grit or corrosion propping open one of the brass compensation port flap valves. This can cause poor fluid draw for bleeding, bypass of pressure and fluid internally from circuit to circuit and slow leakdown within one or both circuits.
Time to open the MC and see what you have. Personally.....if I am replacing or rebuilding ANY of the rubber parts in the system....calipers....wheel cylinders....hoses......the MC gets rebuilt or replaced as well. Its false economy to do otherwise. They all have the same time of rubber inside, are probably all the same age and are exposed to the same chemistry.
If you find very light rust inside the cylinder.....lap it. Do not hone it.
If the rust pits are deeper than ahout 0.003".....the cylinder is shot. You can "try" honing lightly....use a 400 grit or finer hone. I use a 600 grit flexhone. Usually you can take the rust down to about .001" deep pits and the seals witll have no issues sealing on that.
But.....you need to measure the piston heads you have....and then measure the bore. The maximum oversize allowed is 0.005". Typically most MC are already at 0.003" larger than the piston heads. They can only stand removing right at 0.002" of metal. Anything above .005" bore to piston tolerance.....and life of cylinder will be very short.
Ray |
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Become
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Joined: August 25, 2008
Posts: 8
Location: Duluth, GA
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| Posted: Fri Aug 06, 2021 12:14 pm Post subject: Re: Help with a funky brake issue |
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| W1K1 wrote: |
| i had a sinking pedal once, it was a leaky aftermarket brake light switch, just a drip, and the pedal felt good pumped up, but then it would slowly start to fade if you kept pressure on it |
Looks like I found the issue, and it was the brake light switch. Not a leaky aftermarket one, but the center section of the stock two prong switch was blowing out of the housing and causing a super slight drip. I replaced it with a crappy aftermarket one for now and it seems to be working fine. Drove 20-30 miles today in suburbia and the pedal never went soft on me. |
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Bobnotch
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Joined: July 06, 2003
Posts: 22410
Location: Kimball, Mi
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| Posted: Sat Aug 07, 2021 11:15 am Post subject: Re: Help with a funky brake issue |
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| Become wrote: |
| W1K1 wrote: |
| i had a sinking pedal once, it was a leaky aftermarket brake light switch, just a drip, and the pedal felt good pumped up, but then it would slowly start to fade if you kept pressure on it |
Looks like I found the issue, and it was the brake light switch. Not a leaky aftermarket one, but the center section of the stock two prong switch was blowing out of the housing and causing a super slight drip. I replaced it with a crappy aftermarket one for now and it seems to be working fine. Drove 20-30 miles today in suburbia and the pedal never went soft on me. |
I was going to say I've had that issue before on a couple of cars. Even a friend of mine ran into that issue before. Sometimes it's really a matter of looking for the easily over looked stuff first, before digging into the expensive stuff. The switches are cheap to buy, and can fail.
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