back breaks lock up before the front

[Robliepse]

Ill start with a little history. I just bought the 72 GTX RR about a month ago for $13,500 and believe it or not it appears to be almost 100% original. If i had to guess someone found it rusting away somewhere, bought a few cans of bond, painted it back to Torid red and sold it to the previous owner in 2006. I have the E bay add from when he bought it in 2006. Well, over the past 30 days I've confirmed the VINn to the cowling stamp to the radiator support stamp to the broadcast sheet and it is a real 72 GTX and it appears to be numbers matching based on the block, intake and carb numbers. The previous owner also kept every receipt and I've counted close to $10K in suspension, steering, brakes, interior ...........work he paid a local shop to do for him. This isn't my first 72 RR and it's very clear to me that the shop charged way to much and did half *** work. I have one of his $4,800 receipts from last August , yes August 2013, where he paid to have the transmission rebuilt and a complete brake job including lines, rotors, drums, master cylinder.

Now that the history lesson is complete lets talk about the brakes. I've been turning wrenches for years and when I got out of the Army in 1986 my everyday driver for about 7 or 8 years was a 1972 340 Roadrunner. I didn't have any money but a box of tools and I know these cars. The break pedal is firm but you have to really push the pedal to get it to stop. Tonight I was cruising down the road at about 50 MPH, hit the breaks and the back tire locked up with no front end dive. It was to the point I thought they might have connected the lines to the proportioning valve backwards. I verified that this was not the case. My thoughts now are air in the front lines and the back is solid enough to maintain a firm pedal, bad master cylinder, break booster would explain the hard pedal but not the rear locking up.

Sorry about writing the novel but any suggestions would be helpful

 

[dangina]

wow sounds like they were milking this guy -
is it stock brakes? if your running the stock MC chuck that away and get a aluminum one, they are cheap CHEAP upgrade and you save on weight! One area I don't like to cheap out on is the brakes, after all its whats stopping you.

another thought, because its so recent, will the shop warranty their work?

 

[JamieZ]

Are you getting any front brakes at all? If you are I'd try to bleed them and see if you are getting fluid and how much you are getting. It could be the proportioning valve is stuck. They are rebuildable (but you'll have to bleed the brakes after)

 

[miller]

I'm not familar with a 'late model' 72 like yours, but I'd suggest getting your hands on a service manual for it. The manual will have all the needed info for that year model.

Might be something simple, just basic brake servicing. But, Mopar, like all the other makers, went through various stages of brake development. Some of the later brake systems required certain 'added' devices, to make the improved system work.
Might need to look your system over, to know if it's all there, and what might need to be checked.

 

[Robliepse]

Thanks, I do have the service manual so that a good idea. I was looking at it last night to make sure these bozos had the proportioning valve connected properly but I'll take another look at the troubleshooting section. I would take it back to the shop but it 400 miles away The service manual is a great resource. This is actually a pretty simple break system that I've worked on more times than I can remember. I actually swapped the front drums for disk on a 72 Satellite clone project a few years ago.

I'll start with bleeding the front brakes tonight after work. What master cylinder would you recommend Danjena?

Thanks again for everyone's help and support and I added the labor part of the previous owners invoice from 08 / 01/ 2013

 

[dryheat]

If it was a single back tire that locked up take that drum off and check for foreign substances or broken parts.

 

[AMX364]

Pull BOTH rear drums and check for leaking wheel cylinders or axle seal leak first. Check fronts also for grins. I assume front discs? Or is it 4 wheel drum? Or 4 wheel disc? If 4 wheel discs then the work becomes harder and more guessing is involved.

 

[JamieZ]

When you switched from Drum/Drum to Disc/Drum did you switch the Master and Proportioning/Distribution valve? Combination systems need a different master and valve.

 

[Robliepse]

the drum to disk was on another 1972 clone car i was working on and sold a few years ago. This is a real numbers matching 72 GTX with factory front disk and rear drums. I checked and bled the front breaks this evening without any improvement. I'm going to go through the proportioning valve bleeding process that's in the service manual tomorrow. besides the back wheel locking up with very very hard breaking the breaks just don't feel right. The pedal is really hard and firm but the car don't stop. Its hard to explain but they are not right. I'm thinking the proportioning valve is bad but that's an expensive shotgun repair

 

[GearAddict]

the drum to disk was on another 1972 clone car i was working on and sold a few years ago. This is a real numbers matching 72 GTX with factory front disk and rear drums. I checked and bled the front breaks this evening without any improvement. I'm going to go through the proportioning valve bleeding process that's in the service manual tomorrow. besides the back wheel locking up with very very hard breaking the breaks just don't feel right. The pedal is really hard and firm but the car don't stop. Its hard to explain but they are not right. I'm thinking the proportioning valve is bad but that's an expensive shotgun repair

I'm actually having the same problem on my 69 RR right now, except it's the right front drum. The pedal feels stiff as hell and the right front locks up as soon as I put pressure on the pedal. This happened all of a sudden on my first drive out after the car sat for 5 months.

I don't have an answer for you yet, becasue I was already about to start a disc brake swap and figured I'd see the answer in the process of tearing everything down. I'll be working on it this weekend, if something jumps out at me as the culprit, I'll make sure to post it.

I wouldn't think it was the prop valve as I think it is just a static part (brass block machined to create differential pressure. Although I suppose if you had a lot of rust/debris in the lines it could have collected in the prop valve? do you know if both rears locked or just 1? that might tell you it's not the valve, it's in the drum..since there is only a single line feeding both rears.

 

[nm9stheham]

Sounds most likely that the MC piston seal in the front or both sections has gone; it will do what you describe. The pedal will be excessively hard and the rear pressure will reflect back through the bad seal into the front system; the pressures will be the same regardless of the existance of a properly functioning proportioning valve. I have never experienced a proportioing valve going bad but if the seal between F and R blew (which I am assuming has to be there), it would do the same thing.

Also, did they change out the rubber brake lines? It would be unusual for both front brake rubber lines to have the inner liners break up and collapse into the tubes and block the fluid flow in both at the same time, but with the apparent shoddy maintenance, I would take nothing for granted.

I would also diassemble the front calipers from the caliper holders and see if they are floating free on the guides/pins. Too many brake jobs are done without the front guides being cleaned and receiving a thin, fresh coat of brake lube.

When the the back brake(s) locked last night, was this a hard stop or a normal stop? If hard, then that points to a bad front section; if normal, then you more likely have something up with the rear(s). Was there a lot of moisture around last night that could have soaked into the shoes? That can cause certain drum barke linings to lock more easily. (And lots water can also make them feel like they have been greased!)

The drums should have different front and rear shoes (primary and secondary). If these were swapped in the installation, then this WILL cause drum problems galore. Good brake companies will mark primary and secondary but some aftermarket stuff wil lnot have makrings and so can esily be confused. If the shop was not familiar with drums, I can easily see this happening.

 

[bm02tj]

when you see (bad) brake shops use line pinchers on the rubber lines that will causes failures
so anything is possible

 

[nm9stheham]

when you see (bad) brake shops use line pinchers on the rubber lines that will causes failures
so anything is possible

Since I never use them, I did not think of that! You're right; both front rubber lines could be damaged at the same time if they were older lines.

 

[Robliepse]

Thanks MM9, according to the $1K+ receipts from the previous owners Madison Automotive in Madison AL (Suburb of Huntsville) changed everything but the booster in August 2013. Yes, less than a year ago. I was bleeding the front breaks a couple of days ago and the left caliper had a 3/16 bleeder valve that was so tight it almost broke off using a 5 point socket. the right side had a 5/16th bleeder valve that was a tight and worked as suspected. Then I noticed that the clips that secure the rubber front lines to the steal lines were missing one both sides. Oh and it get better, there were only 3 of the for nuts holding on the Master cylinder. Great top dollar break job. There appears to be good vacuum and I tested the breaks with and without the vacuum hose with no difference. I'm going to change the booster and the master cylinder on Monday. Either they put on the wrong master cylinder or the replaced everything but the booster which might have been the only thing wrong. The only other thought is they put the wrong rear wheel cylinders on the car. If the rear ones are too small it will throw the entire system off.

- - - Updated - - -

wow terrible spelling, For not Four and breaks instead of brakes...............sorry

 

[nm9stheham]

But small rear wheel cylinders will put less force on the rears, not more......I'd like a picture of your 5 point socket please! LOL Hope to hear good news back on your 'breaks'....

 

[sleepar]

I have a receipt for $2000 worth of brake conversion that never made it on the car the shop must have had someone who worked there with a mix of parts .... anyway go through it again New rubber hoses at a minimum check the whole system throughout

 

[Robliepse]

thanks for all the input. The car is on jack stands with all 4 tires removed and I'm going to go through the entire system starting tomorrow after wok. I removed the back drums tonight and all new parts but something just didn't seem right. Might be the primary , secondary shoe issue MM9 discussed above.

 

[Robliepse]

Update on fixing the previous owners $1,000+ shop brake job. Based on the pictures in the chassis service manual the rear primary and secondary pads were installed correctly. I easily installed the new AutoZone booster and master cylinder although I have a receipt that said the master cylinder was replaced in August 2013. I also performed the proportioning valve reset procedure that's in the service manual. Bleed front and back pretty good, opened both front bleeder valves and aggressivly worked the pedal to simulate a front break failure which is supposed to reset the valve. Next I re bled the breaks using the old pump and hold method. The brakes are about 200% better than before but I think only about 80% as good as they can be. I noticed a good bit of rust and debris and nasty brake fluid coming from the back brake bleeders even though i have the receipt and there appeared to be new rear wheel cylinders and new steel lines back to a new t-fitting on the rear axle. I also noticed some rust colored brake fluid on the lines directly from the master which makes no sense. I was going to completely remove and proportioning valve and clean all the lines but the fitting were not coming off and I'd by replacing all the lines.

My theory is that the booster was bad or very week and old (bad is bad i guess) because I can now tell the booster is assisting with the brake function. I also believe that the proportioning valve was way clogged with the old dirty rusty brake fluid. I think the remaining 20% that's still not correct is still associated with the junk in the valve so I plan on giving it a very more miles and then rebleed the brakes.

 

[nm9stheham]

Good news to hear! All of the above is why I try to do as much of my own work as I can; the few times I take a car/truck anywhere, seems like it is a 50-50 chance of something being hosed up. Hard to believe that for all those $$, there was still rusty brake fluid in the system.

So is the rear lock-up solved?

I'd rebleed the lines in a few days or weeks just to keep flushing all the crap out; sound like there could be moisture in the system that is causing the continued rust. You need to get all of that out.

BTW, I forgot to add earlier; you can test a booster by: let the car sit for a while. Apply the brakes pedal no more than medium hard; start the car and when it fires, the pedal should sink further as the booster gets vacuum and activates.

 

[Robliepse]

Thanks MM9, I unhooked the vacuum from the booster, plucked the hose with a bolt and took a quick ride and found very little difference in braking with and without vacuum. I guess your method is safer. Went down to a well respected local alignment shop today for an alignment and to no surprise they told me the entire front end needs rebuilt. I have receipts from the same shop that did the brakes that they replaced some parts but not all last summer. Putting her up on the jack stands this evening to start the rebuild.