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Hey all. I don't have a Corvette but I've got a project truck that's got a Corvette c3 master cylinder on it and I'm trying to diagnose a weird issue. This truck has Chevy C10 calipers in the front and the rear, all identical. I have no proportioning valve no bias valve no combo valve etc. I have a line to the front brakes from the front of the master cylinder, and a line to the rear brakes from the rear of the master cylinder. I get very little response from the rear brakes unless I pump the pedal two to three times coming to a stop. I have replaced the master cylinder bench bled it, bled the whole system over and over and I seemingly don't have any air in it. And I get a pretty decent flow when I open the rear bleeders. So my question, on a stock c3 master cylinder, is the flow from the front and the rear ports identical? Or is the flow from the rear port, for the rear brakes less than the front to add some kind of bias. I know stock c3 Corvettes had a junction block with a warning light switch in it, and then a proportioning valve on the rear brakes. But I don't know if the master cylinder delivers the same flow from the front and the rear. If I crack the lines on the master to bleed the master it seems like I get a lot more fluid coming out of the front circuit than I do the rear for the same amount of pedal travel which does not make a lot of sense to me. I'm trying to eliminate the possibility of two bad master cylinders. Let me know what ya think, thanks!!
Yeah, it's got a hydro boost. Pushrod is adjusted properly. Even if it wasn't I'd have excessive pedal travel, but the truck would stop evenly, not dive in the front. And I couldn't be able to pump the pedal 2 or 3 times and have the truck stop evenly. This is an air still trapped somewhere or a flow issue with the master. In an effort to try and eliminate the master as being totally wrong for the system, this is why I asked the flow question for the c3 master
The C3 MC doesn't bias the pressure front to back. On the early C3 the bias was different because of different calipers front to back,,,,,,,, "NO PROPORTOINING" valve. On the later C3 there was a real proportioning valve. So either way the MC doesn't bias,,, front to back.
Ok, the master doesn't bias the flow...that's what I needed to know. This master has different size ports front and rear so swapping the lines would not be easy. Far as I knew all c3s has different size pistons front and rear to deal with the bias. But anywho...I'll probably pull the master off again to bet sure it's completly free or air. Then I should stop assuming all the pistons in the calipers on the truck are the same size and actually measure them to be sure. This is one of those vehicle where it's a hodge lodge of parts, but decently built, but didn't run or drive when you got it, now your fixing all of its quirks.
It does seem strange. If I bleed the master by pushing the pedal and cracking a line at the master, I get more fluid from the front line than I do the rear line. Like I said I'll pull it off again to bench bleed...but just seeing the difference In fluid, this is my issue for sure. It would be worth, though not unheard of to have an old and new master do exactly the same thing. Cause this is a new off the shelf master.
Well, yes & no about the MC port sizes. The brass fittings are different sizes. Maybe a 1/2 & 9/16 ? Don't remember. But the brake lines are the same diameter so that's what counts.
If the MC rear port pressure / seepage is weak, it could be faulty rear seal on the rear piston only.
I question if your bench bleeding process is the best method, or you maybe damaging the rear piston seal ("O" ring) if done incorrectly.
What happens often is, the MC piston is over extended in the bore while bench bleeding. The piston ring snags on the castings causing the seal to roll over on the return stroke, rendering at least one of the piston seals useless.
This is why good MC sellers / rebuilders label the parts box with a warning: Do Not Force Piston In More Than 1 & 3/8 inches. Or voids warranty.
This MC damaging scenario can not happen once the MC is mounted on the vehicle. There is a stop to prevent over-extending. Its called the floor.
But on a bench vise, it can happen and does.
See my Profile> Photo Album> Brake Bleeding for more info / photos.
The only way your MC will get damaged when bench bleeding, is, if it is all rusted to hell inside the bore. If that is the case it should be replaced. If it is a new shinny MC, bench bleed the heck out of it. It is also a good idea to bleed it level at first,,,, then tilt it a bit, back end down, then back end up. Watch the bubbles, you will get more air out of it. Make sure you are using clear tubes.
I have been reading about old chevy trucks,,,,,,,,,,,, It is supposed to have a real "PROPORTIONING" valve on the brake system. This will help to bias the brakes front to back. With just the corvette MC that won't happen.
ALSO,,,,,, if you can pump the pedal and get better brakes, you have air in your system.
As a general rule,,,,, you are correct. If you are bleeding brakes on an old car, you need to be very careful not to push the cylinder to far into the bore or you can ruin the seals or push a bunch of crap into your system. Now you have created a bunch of other issues with your entire brake system. Any shop would use this method to ensure problems don't happen. If you have a new or rebuilt MC and know the bore is clean and clear, not a worry.
As a general rule,,,,, you are correct. If you are bleeding brakes on an old car, you need to be very careful not to push the cylinder to far into the bore or you can ruin the seals or push a bunch of crap into your system. Now you have created a bunch of other issues with your entire brake system. Any shop would use this method to ensure problems don't happen. If you have a new or rebuilt MC and know the bore is clean and clear, not a worry.
If your statement were true, then why does the MC manufacture state: Over extending the piston travel will void the warranty. Do Not Force Piston More Than 1 3/8.
This was the warning label right on the brand new MC box in big yellow & black print.
I don't know about you, but to me and thousands of other customers, you better heed this warning or damage will occur. They should know.
Has nothing to do with rust in a brand new bore.
When the MC is in a vise on the bench, there is no limit on how far someone shoves in the screwdriver & piston.
On the car, the brake pedal will hit the floor / carpet, limiting its travel.
Don't get me wrong,,,,, You are correct,,, there are thousands of master cylinders, and many have this issue, I am just talking about the original C3 MC.
Yes, there is a limit. On the end of the shaft there is a spring and a rod that limit its travel. There is nothing in the bore to catch the lip of the seal. In fact during normal operation, the seals are pushed past the reservoir holes on every application of the brakes.
If I crack the lines on the master to bleed the master it seems like I get a lot more fluid coming out of the front circuit than I do the rear for the same amount of pedal travel which does not make a lot of sense to me. I'm trying to eliminate the possibility of two bad master cylinders. Let me know what ya think, thanks!! Mark
Some of the M/C bleed kits have two plugs included in the kit or you can buy two M/C lines, one with a 9/16 18 fitting and the other with a 1/2 20 fitting. and crimp the lines to make a plug.
To test the M/C on the car:
Remove 1 line and plug it and note the brake pedal travel.
Then reinstall the first line and plug the other one,
The pedal should be rock hard in both tests, if not the front or rear seal is bad.
So I won't discount that the master cylinder could potentially be damaged by overextending during bench bleeding. In my experience I have never had this happen, and I've installed many many master cylinders in the service industry. Now maybe there's a particular manufacturer that uses certain parts, or maybe there's certain master cylinders where this might be more likely. But in a brand new master cylinder, where there's nothing to catch seals on, no stepped bore, how am I going to roll a seal, this seems very confusing to me. Now in the field I have seen people bleed brakes and press the pedal to the floor and have pistons get stuck in the master cylinder due to crud stuck in the master, or seal damage. When I was first starting to wrench an old timer taught me when you're bleeding brakes on a car with an old master cylinder don't push the pedal more than 2/3 the way down for exactly this reason. And I've used this technique when bleeding brakes on a car that I haven't replaced the master cylinder on and never had an issue, obviously good advice. But like I said, in a new master cylinder what is going to make that lip seal roll over? Obviously it is a problem if some manufacturers are putting on the box don't press more than x inches. But I didn't see that warning on this box. Unfortunately when I bench bled this master I didn't have a vise to put it in, so my buddy held it while I push the piston and held my fingers over the holes did this a few times and that was that. It is quite possible that there is still air trapped in the master because of this, but I'm doubtful. And the mention of proportioning valve, well I appreciate and understand what the valve is there for, in this case I have no proportioning valve in the car, because I'm trying to make all four wheels stop evenly before trying to bias the rear brakes. Having an extra part in there to limit pressure to the rear brakes, when I have a rear brake pressure problem to begin with seems redundant. And an effort in trying to eliminate potentially defective parts, like a defective rear biasing valve, I removed it from the system and eliminated that from being an issue. So until I get proper brake pressure on all four wheels and the truck stopping evenly, I won't be putting my adjustable rear bias valve back in. I am curious exactly what master cylinders have you seen aka what applications / vehicles where there is a warning about how far to push in the piston when bench bleeding. Also I have attempted bleeding the master on the vehicle pumping the pedal pushing it all the way to the floor with no better results in this particular case. Unfortunately it may be a couple weeks before I get back to this vehicle, but I like the conversation so far
Unfortunately when I bench bled this master I didn't have a vise to put it in, so my buddy held it while I push the piston and held my fingers over the holes did this a few times and that was that. It is quite possible that there is still air trapped in the master because of this, but I'm doubtful.
So I won't discount that the master cylinder could potentially be damaged by overextending during bench bleeding. In my experience I have never had this happen, and I've installed many many master cylinders in the service industry. Now maybe there's a particular manufacturer that uses certain parts, or maybe there's certain master cylinders where this might be more likely. But in a brand new master cylinder, where there's nothing to catch seals on, no stepped bore, how am I going to roll a seal, this seems very confusing to me. Now in the field I have seen people bleed brakes and press the pedal to the floor and have pistons get stuck in the master cylinder due to crud stuck in the master, or seal damage. When I was first starting to wrench an old timer taught me when you're bleeding brakes on a car with an old master cylinder don't push the pedal more than 2/3 the way down for exactly this reason. And I've used this technique when bleeding brakes on a car that I haven't replaced the master cylinder on and never had an issue, obviously good advice. But like I said, in a new master cylinder what is going to make that lip seal roll over? Obviously it is a problem if some manufacturers are putting on the box don't press more than x inches. But I didn't see that warning on this box. Unfortunately when I bench bled this master I didn't have a vise to put it in, so my buddy held it while I push the piston and held my fingers over the holes did this a few times and that was that. It is quite possible that there is still air trapped in the master because of this, but I'm doubtful. And the mention of proportioning valve, well I appreciate and understand what the valve is there for, in this case I have no proportioning valve in the car, because I'm trying to make all four wheels stop evenly before trying to bias the rear brakes. Having an extra part in there to limit pressure to the rear brakes, when I have a rear brake pressure problem to begin with seems redundant. And an effort in trying to eliminate potentially defective parts, like a defective rear biasing valve, I removed it from the system and eliminated that from being an issue. So until I get proper brake pressure on all four wheels and the truck stopping evenly, I won't be putting my adjustable rear bias valve back in. I am curious exactly what master cylinders have you seen aka what applications / vehicles where there is a warning about how far to push in the piston when bench bleeding. Also I have attempted bleeding the master on the vehicle pumping the pedal pushing it all the way to the floor with no better results in this particular case. Unfortunately it may be a couple weeks before I get back to this vehicle, but I like the conversation so far
When I bleed the C3 MC I also use long strokes. But it takes many many many pushes to get all the air out. Even after bench bleeding the hell out of it,, and installing on the car I still had air trapped in the MC. There are still a few things to do once it's back on the car.
Sounds like you need residual pressure valves at each of the rear calipers. The 2-3 pumps to get the rear brakes to work sounds the pistons are fully retracting after releasing the brakes.
08-26-2021, 05:06 PM
Question about C3 master cylinder flow
