Converted to dual master with disc/disc. MC is corvette style without residual. MC is mounted high. Proportioning valve installed in rear line, wide open now until I can adjust. I have a couple hundred miles on new brake assembly. Brakes work great until I drive and things get hot, 15 mins of fun, then brakes start dragging hard. Notice that pedal is not returning fully. After cools down, behaves okay for a bit. Checked clearance on rod between booster and MC, it has proper RCH. Put MC on bench, and while bench bleeding I notice that piston does not retract all the way to the clip. If I plunge all the way, and let go sharply, piston retracts fully. If I plunge all the way, and slowly come out, piston stops with 1/3 travel to go. Found the problem, but sort of weird... why is it happening? It sat for a few years with fluid, bled, functioning brakes. MC was purchased new, not rebuilt. It's the type for hot rods with ports on both sides of MC. Sooo... I figure I need to run a hone in the bore. Clip comes out, and first spring and seals come out. The second piston remains in there. If I plunge on it, I can see that is the one with the bad behavior of retract fully if let go sharply, and stops short of I retract slowly. Questions: - How do I remove it? I can't get any tools deep enough to grab the tit. - Does it sound like a hone will fix the problem? Thx.
I'm sort of wondering if this actually designed for disc/drum, and the second port has a residual valve in it? Second port is 1/2" which is typically rear, and first port is 9/16". I bought it from: http://www.partscityusa.com/store/mastercylinders.html Which lists it as disc/disc. But, it looks exactly Speedways, which lists it as disc/drum: http://www.speedwaymotors.com/GM-Corvette-Cast-Iron-Master-Cylinder-1-Inch-Bore,51673.html So, if the second port is actually for drum, would I have the problems I'm describing?
sounds like you found your problem. i always like finding something wrong. apossed to not finding any thing. sound like the plunger might be broken in half??? try to look in the hole fron the top, can you use air to blow it out or is the plunger in to far? you might just pop for a new MC. altho they cost more than a rebuild kit, the kits are $$$$ compared to a MC
Make sure your pedal travel is returning far enought to allow fluid pressure to be released with your foot off pedal. I had the very same thing happen on the divco. Had to adjust the lenght of the push rod. When the fluid got hot it expanded and applied the brake just like pushing the pedal. After sitting for as little as 10 minuites, brakes released. If your pedal has freeplay when cold and no freeplay when hot, the pedal is not allowing the piston to retract enought to allow the piston to release line pressure back into the master clylinder ! .
Yes, I thought it was rod clearance initially, but the problem even happens on the bench with only MC. See video. It retracts with sharp plunges, but on final plunge, slowly pull out and piston stays in ~1/2"-3/4". Bleeder kit is on, it is returning fluid to reservoirs. http://www.youtube.com/watch?v=7wA1puPqoDQ&feature=youtu.be
To get the piston out you might try smacking the m/c on a 2"x6" and see if inertia will push it out.... But, I like the compressed air idea better...
The residual pressure in the lines should be more then enough to return the pistons. The orifice is less then .100 thousands of an inch. ( less then 1/8 of an inch) It does not take much to restrict the return flow to the reservoir. ( reliveing line pressure) You can put a temporary shim/spacer between the fire wall and the master cylinder. Cut a slot in two very thick washers. It took less then an 1/8 of an inch, to fix mine. Mine was easy to adjust, I had made mine, adjustable. Do you still have the old cylinder, compare the depth of the piston from the mounting flange. Wether it is a disc/disc or a disc/drum, it does not matter it still has to clear the orifice on the return of the pedal. .
carefully push a small drill bit into the ports. If there's a RPV in there you will feel it. It will resist a little but with a little more pressure you'll overcome the spring. If you did have a drum MC you probably would have this problem, but I would think it would happen sooner than 15 mins into a drive.
The problem just may be when the fluid get hot it expands rapidly, as the brakes are dragging generating every hot temps. After cooling down brakes will retract enough, to imply all is ok. This was the problem with mine. .
I know you said that you bench blead it and it has been on the car but did you bench bleed both sides ? I've had similar issues where a customer capped the unused ports and bleed only the ports used trapping air in the master cylinder.
probed with drill bit, no valve. thanks for tip. compressed air got it out, thanks for idea on that. just had to set up a firing zone in case it came out like missile. i honed quite a bit. while the deep piston is much smoother than it was, it still resists retracting all the way. i'm finished with this MC. it should be very smooth without any troubles like this. i agree, it makes sense to just buy another MC, but i use the inboard side of ports so i need this crazy dual port MC that is mail order, and is it just going to come from same mfg that is giving me headaches? i think my solution is going to re-plumb the lines to the outboard ports so i can use the normal corvette disc/disc setup that is available everywhere. just need to figure how to clear the tight fit with wheel well. Spanky used to say "There's always something..."
that's interesting, and a great point with these dual port MCs. i suppose one solution would be to bench bleed with the MC listed to the side so any air comes up from plugged ports.
It is simple, is there free play in the pedal ? Can the push rod be disconnected, from the pedal, during the problem ? I mean, not having to pull on the pedal to diconnect it.
right, i understand. as i mentioned - that was my first thing to check. and i checked it thoroughly. clearance is great on everything. the problem is that the piston in the MC will not smoothly retract. check out the video. problem is isolated to the MC. thanks for suggestions.
Just as a note I did a Kelsey Hayes SSBC knock-off disc conversion on a 68 Mustang last week, even though it did not apply to this setup it boldly stated it must be done on masters such as yours. In the original case the master was replaced twice, before he gave up and brought it in. I also had a 75 Corvette with a similar issues a few months back. After a few trips back to the brake shop the owner asked us to look at it while we were touching up the body. The dual chamber calipers were only bleed off the inside and as it would heat up it would loose pedal. No matter what the actual problem is, just buying a new one doesn't teach you anything. Even if it comes to that because of the cost of repair or available equipment to make the repairs, figuring out what the root of the problem is valuable information.
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