What would happen if the master cylinder is almost filled to the top?

[TWINRAY]

Instead of leaving a quarter inch or so of space? I have a reason for asking but don't want to sway any answers. Thanks, Fred

 

[JonM]

I would think it would make a mess every time you pressed on the brake due to no available expansion. And if it didn't make a mess, it might brake a seal someplace in the MC or caliper due to pressure.

 

[ßill]

The worst thing that will happen is a little overflow when you put the cap on. The good thing is no air on top of the fluid. Brake fluid (except silicone based) will absorb moisture out of the air, no air no water. So wipe up the overflow when you put the top on, Then smile.

 

[TWINRAY]

Well, this is what happened. After doing new rotors, pads, front wheel bearings and resealing calipers this weekend, I bled the system with the Motive Bleeder. After doing so, it left the master pretty well filled up and I left it as is and put the cap on. Drove the car for 3 days this week round trip (8 miles each way) to work and all was OK. Today, about half way in to work, it felt like I wasn't moving as quick as I should. I then realized:ugh I had almost no free play in the brake petal. Good thing I reached work when I did cause I think the Fkr would have stopped in its tracks if I drove any further. It dawned on me that maybe the filled up master was causing the problem. I took some fluid out and left about 1/4" free space. took a couple of blasts around the block and it's OK now. Think it could have been too much fluid not leaving any room for expansion? But I wonder why it was OK for Monday - Wed?

 

[gmjunkie]

The worst thing that will happen is a little overflow when you put the cap on. The good thing is no air on top of the fluid. Brake fluid (except silicone based) will absorb moisture out of the air, no air no water. So wipe up the overflow when you put the top on, Then smile.

Bingo!! junk!!

 

[wishuwerehere82]

Well, this is what happened. After doing new rotors, pads, front wheel bearings and resealing calipers this weekend, I bled the system with the Motive Bleeder. After doing so, it left the master pretty well filled up and I left it as is and put the cap on. Drove the car for 3 days this week round trip (8 miles each way) to work and all was OK. Today, about half way in to work, it felt like I wasn't moving as quick as I should. I then realized:ugh I had almost no free play in the brake petal. Good thing I reached work when I did cause I think the Fkr would have stopped in its tracks if I drove any further. It dawned on me that maybe the filled up master was causing the problem. I took some fluid out and left about 1/4" free space. took a couple of blasts around the block and it's OK now. Think it could have been too much fluid not leaving any room for expansion? But I wonder why it was OK for Monday - Wed?

It was warmer today than it has been the last few days.
Brake fluid expands with temperature and leaving the reservoir with some air space allows the fluid to backflow to the master cylinder when the pedal is released and when the fluid expands. If it didn't have any room to flow into the reservoir, the pressure would be maintained in the brake lines as you found out. You learned a valuable lesson, the hard way, but not too hard.

 

[goertz]

can you please put a shirt on?

 

[TWINRAY]

It was warmer today than it has been the last few days.
Brake fluid expands with temperature and leaving the reservoir with some air space allows the fluid to backflow to the master cylinder when the pedal is released and when the fluid expands. If it didn't have any room to flow into the reservoir, the pressure would be maintained in the brake lines as you found out. You learned a valuable lesson, the hard way, but not too hard.[/QUOTE

Something is still fashimmal. I replaced the front brake hoses cause I thought I may have fkd them up upon caliper removal. Rebled and left free space in the master and took her to work this AM. About 5 miles out noticed not quite as bad a situation as before but free play greatly reduced. Stopped and got gas (@ $3.359/gallon). Started it up and drove the remaining 3 miles. Had a bit more free play. Got to work and still had clearance in the master but thought it maybe expanded a bit too much and brought the gap down to the lip. If I can't get this sorted out by next weekend for the Watkins Glen event, it's either taking the wifes '79 Camaro instead or (which I don't really want to do) putting the old rotors and pads back in. Last call for advice:confused

 

[TWINRAY]

Spoke to VBP tech support

suggested I take one turn out of each master cylinder to booster nut. Also maybe take a half turn -turn out of the push rod in the booster. If it still is NG, then it probably is the seals in the master that swell up which means either a new master or a rebuild of the old.

 

[wishuwerehere82]

suggested I take one turn out of each master cylinder to booster nut. Also maybe take a half turn -turn out of the push rod in the booster. If it still is NG, then it probably is the seals in the master that swell up which means either a new master or a rebuild of the old.

It does sound like a MC problem. The piston should return back all the way with no pedal pressure and allow the pressure in the system to equalize. Even if the calipers are sticking the engagement point on the MC should start applying pressure at the same point every time. Sounds like you are right, it could be the seals on the MC swelling and causing your engagement point to change as you work the cylinder.
Good luck!

 

[gmjunkie]

It does sound like a MC problem. The piston should return back all the way with no pedal pressure and allow the pressure in the system to equalize. Even if the calipers are sticking the engagement point on the MC should start applying pressure at the same point every time. Sounds like you are right, it could be the seals on the MC swelling and causing your engagement point to change as you work the cylinder.
Good luck!

Very Poss.! I'd Say it's the M/C, you shoudn't have to ajust rod on Booster!!junk!!

 

[TWINRAY]

I turned out by 1/2 turn the bolts holding the master to the booster. Agreed, I shouldn't have to mess with the adjusting rod. The master is still snug to the booster even after the 1/2 turn out. If there is ANY hint of loosing free play on the ride home tonight, I'll just replace the master. Thanks, Fred

 

[Mr. Chuck]

Most MC that I have seen have a hot / cold fill line on the reservoir, fluid levels between these limit should eliminate the expansion issue. To check if calipers are releasing properly, with car off the ground so you can manually spin each wheel, have a helper pump up the brakes, release and spin each wheel by hand to see if fluid pressure is gone from the respective caliper. If you find all four locked, then your MC is suspect. Also insure that brake peddle linkage is free and not binding. Good Luck!

 

[RodsnRides]

Mr. Chuck -

Fair enough on the latter part, but the Master Cylinders I have dealt with don't have a hot/cold line (?), more like high/low-if they are plastic vs. cast MC's.

I agree with the wheel spinning and checking for linkage bind, however. I try to stick with the KISS theory (Keep It Simple...).

It may be as simple as lubing the linkage points with lithium grease or a shot of WD-may also check for caliper bind. Best,

Rick

 

[TWINRAY]

Mr. Chuck -

Fair enough on the latter part, but the Master Cylinders I have dealt with don't have a hot/cold line (?), more like high/low-if they are plastic vs. cast MC's.

I agree with the wheel spinning and checking for linkage bind, however. I try to stick with the KISS theory (Keep It Simple...).

It may be as simple as lubing the linkage points with lithium grease or a shot of WD-may also check for caliper bind. Best,

Rick

Correct, older vehicles don't have min/max line markers. I ordered a new master and will pick it up tonight. I can't dik around with this any longer. Sometimes it good, other times not. I'll at least eliminate one variable. Thanks