moose425 16 Report post Posted January 29, 2012 I have a 1969 mustang with power disc front brakes, and power drum rear brakes. I have replaced the master cylinder, and the distribution block valve as well as the proportioning valve. The distribution block valve and the porportioning valve were rebuilt by the seller. I have installed all and bled the brakes, now my rear brakes are locking up under moderate pedal pressure. What is the solution to this problem? Could it have something to do with the adustable brake plunger on the vacuum booste? I turned it out a couple of turns before installing the master cylinder as I was getting excessive pedal travel with no braking effect. Thanks so much for any help, MOOSE Quote Share this post Link to post Share on other sites
Fordrevhead 29 Report post Posted January 30, 2012 Show us a photo or better yet diagram of the dist blk and prop valve plumbing. Quote Share this post Link to post Share on other sites
Cruzzar 22 Report post Posted January 30, 2012 Are you sure that the front brake line is plumbed into the front brake port on the master cyl. and not reversed? Just a wild guess. Quote Share this post Link to post Share on other sites
stangs-R-me 121 Report post Posted February 1, 2012 (edited) I'm guessing the piston in the distribution block is either shifted forward (towards front brake side) or the seal on the back end of the spool that separates the far rear to middle ports is bad. With either the spool shifted forward or this seal bad, fluid by-passes the proportioning valve which gives you full pressure to the rear brakes. Proportioning valve could also be bad. You stated the previous owner had re-built the dist block & prop valve and this would be the only "stock" solution as the '69 parts are not available new. Muscle Car Research sells a seal kit for the dist block and a seal & spring kit for the prop valve. But, rebuilding these 43 year old parts with these two kits does not guarentee "like new" performance as some are so bad from years of moisture in the brake system (brake fluid attracts moisture like a sponge) they are not rebuildable. Doug Edited February 1, 2012 by stangs-R-me Quote Share this post Link to post Share on other sites
Fordrevhead 29 Report post Posted February 1, 2012 Are you sure that the front brake line is plumbed into the front brake port on the master cyl. and not reversed? Just a wild guess. Thats why I suggested a diagram. IF its plumbed right, some of the other suggestions are next. Quote Share this post Link to post Share on other sites
moose425 16 Report post Posted February 7, 2012 Sorry it took me so long to respond. Attached is how my distribution block and proportioning valve is installed. Entire system has been bled, and no leaks. Pedal is traveling just about all the way to the floor and rear brakes lock up. Getting good pressure at all ports when bleeding. Thanks so much for any further help. Mark[ATTACH]10449[/ATTACH] Quote Share this post Link to post Share on other sites
Mustangdug 10 Report post Posted February 8, 2012 I have a similar issue on mine, I have 4 wheel disks and my fronts lock up after driveing 5-6 miles. I have to relieve the line pressure at the MC to unlock them, then I can drive around all day with out a problem. My pedal is also sensitive, doesnt take much to stop the car. I have eliminated the Prop valve and installed a adjustable one just on the rear. Still does it. Im going to swap MC's next with a different type amde for disk/disk but i think it's the booster pushing the MC too far and it wont release pressure. I'd be interested to know how you fix yours! Doug Quote Share this post Link to post Share on other sites
Lemon Owner 11 Report post Posted February 9, 2012 mustangdug, your problem sounds like the adjustment of the little bolt on the end of the pushrod to mastercylinder. Quote Share this post Link to post Share on other sites
Mustangdug 10 Report post Posted February 10, 2012 mustangdug, your problem sounds like the adjustment of the little bolt on the end of the pushrod to mastercylinder. So is it too long I would assume? Quote Share this post Link to post Share on other sites
Mustangdug 10 Report post Posted February 10, 2012 mustangdug, your problem sounds like the adjustment of the little bolt on the end of the pushrod to mastercylinder. So is it too long I would assume? Quote Share this post Link to post Share on other sites
Mustangdug 10 Report post Posted February 10, 2012 mustangdug, your problem sounds like the adjustment of the little bolt on the end of the pushrod to mastercylinder. So is it too long I would assume? Quote Share this post Link to post Share on other sites
stangs-R-me 121 Report post Posted February 10, 2012 I have a similar issue on mine, I have 4 wheel disks and my fronts lock up after driveing 5-6 miles. I have to relieve the line pressure at the MC to unlock them, then I can drive around all day with out a problem. My pedal is also sensitive, doesnt take much to stop the car. I have eliminated the Prop valve and installed a adjustable one just on the rear. Still does it. Im going to swap MC's next with a different type amde for disk/disk but i think it's the booster pushing the MC too far and it wont release pressure. I'd be interested to know how you fix yours! Doug I'm thinking your problem may be more related to the fact it sounds like you are using a disc/drum master and NOT using a correct disc/disc type. There is a definitely a difference that makes them NOT interchangable ... get the correct master in there AND verify that the push-rod is adjusted to spec. the other Doug Quote Share this post Link to post Share on other sites
stangs-R-me 121 Report post Posted February 10, 2012 I have a similar issue on mine, I have 4 wheel disks and my fronts lock up after driveing 5-6 miles. I have to relieve the line pressure at the MC to unlock them, then I can drive around all day with out a problem. My pedal is also sensitive, doesnt take much to stop the car. I have eliminated the Prop valve and installed a adjustable one just on the rear. Still does it. Im going to swap MC's next with a different type amde for disk/disk but i think it's the booster pushing the MC too far and it wont release pressure. I'd be interested to know how you fix yours! Doug I'm thinking your problem may be more related to the fact it sounds like you are using a disc/drum master and NOT using a correct disc/disc type. There is a definitely a difference that makes them NOT interchangable ... get the correct master in there AND verify that the push-rod is adjusted to spec. the other Doug Quote Share this post Link to post Share on other sites
stangs-R-me 121 Report post Posted February 10, 2012 I have a similar issue on mine, I have 4 wheel disks and my fronts lock up after driveing 5-6 miles. I have to relieve the line pressure at the MC to unlock them, then I can drive around all day with out a problem. My pedal is also sensitive, doesnt take much to stop the car. I have eliminated the Prop valve and installed a adjustable one just on the rear. Still does it. Im going to swap MC's next with a different type amde for disk/disk but i think it's the booster pushing the MC too far and it wont release pressure. I'd be interested to know how you fix yours! Doug I'm thinking your problem may be more related to the fact it sounds like you are using a disc/drum master and NOT using a correct disc/disc type. There is a definitely a difference that makes them NOT interchangable ... get the correct master in there AND verify that the push-rod is adjusted to spec. the other Doug Quote Share this post Link to post Share on other sites
Lemon Owner 11 Report post Posted February 11, 2012 Yes the rod is too long, and like stangs-R-me said, check you have the correct master cylinder. Quote Share this post Link to post Share on other sites
Lemon Owner 11 Report post Posted February 11, 2012 Yes the rod is too long, and like stangs-R-me said, check you have the correct master cylinder. Quote Share this post Link to post Share on other sites
danno 128 Report post Posted February 12, 2012 The factory system had a problem with rear brakes locking in a panic stop, which is what my car does. It is a bad proportioning valve. Your problem, Moose, could be related but still a few questions remain. The pedal going down that far is a mystery. If you slowly push the pedal down, does it still lock the rear brakes? If it operates better then you press the pedal slowly, then it is the proportioning valve. Is it possible the rear brakes are adjusted to be too far out. When you lift up the rear wheels with the car off and tranny in neutral, you should be able to spin the rear wheels. If not, they could be adjusted too far out. Quote Share this post Link to post Share on other sites
danno 128 Report post Posted February 12, 2012 The factory system had a problem with rear brakes locking in a panic stop, which is what my car does. It is a bad proportioning valve. Your problem, Moose, could be related but still a few questions remain. The pedal going down that far is a mystery. If you slowly push the pedal down, does it still lock the rear brakes? If it operates better then you press the pedal slowly, then it is the proportioning valve. Is it possible the rear brakes are adjusted to be too far out. When you lift up the rear wheels with the car off and tranny in neutral, you should be able to spin the rear wheels. If not, they could be adjusted too far out. Quote Share this post Link to post Share on other sites
moose425 16 Report post Posted February 19, 2012 I have now replaced my proportioning valve with a manual valve (SBBC corp), I have properly plumbed it in. My metering block is properly centered and locked in place with a lock "key" inserted in the electircal switch port. I have a new Tuff-Stuff chrome plated master cylinder installed. When installing the new master cylinder I was able to see piston movement; I adjusted the plunger on the booster until I could no longer see any piston movement at installation. I filled and vented the system at all wheels; and still my pedal is going to the floor!!! I just can't figure this out. HELP PLEASE!! Is there a special procedure I should follow for venting the master cylinder; I have just been venting through the wheel vents. Is there a special position I should put the proportioning valve in when venting? I had it at about mid-point. Need advice! Thanks. Quote Share this post Link to post Share on other sites
moose425 16 Report post Posted February 19, 2012 I have now replaced my proportioning valve with a manual valve (SBBC corp), I have properly plumbed it in. My metering block is properly centered and locked in place with a lock "key" inserted in the electircal switch port. I have a new Tuff-Stuff chrome plated master cylinder installed. When installing the new master cylinder I was able to see piston movement; I adjusted the plunger on the booster until I could no longer see any piston movement at installation. I filled and vented the system at all wheels; and still my pedal is going to the floor!!! I just can't figure this out. HELP PLEASE!! Is there a special procedure I should follow for venting the master cylinder; I have just been venting through the wheel vents. Is there a special position I should put the proportioning valve in when venting? I had it at about mid-point. Need advice! Thanks. Quote Share this post Link to post Share on other sites
Fordrevhead 29 Report post Posted February 19, 2012 You need to bench bleed the new master! This takes some time but you absolutely have to do it. Quote Share this post Link to post Share on other sites
Fordrevhead 29 Report post Posted February 19, 2012 You need to bench bleed the new master! This takes some time but you absolutely have to do it. Quote Share this post Link to post Share on other sites
Kiwi John 10 Report post Posted February 20, 2012 Are your rear brake shoes adjusted up snug so that the drums just drag when turned? To me, this seams to be a possibility. Quote Share this post Link to post Share on other sites
Kiwi John 10 Report post Posted February 20, 2012 Are your rear brake shoes adjusted up snug so that the drums just drag when turned? To me, this seams to be a possibility. Quote Share this post Link to post Share on other sites
Lemon Owner 11 Report post Posted February 20, 2012 you need to bench bleed the new master! This takes some time but you absolutely have to do it. x 2 Quote Share this post Link to post Share on other sites
