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| Spongy pedal on a disc brake swap https://mail.slantsix.org/forum/viewtopic.php?t=61212 |
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| Author: | The Moffittman [ Fri Jun 02, 2017 7:37 am ] |
| Post subject: | Spongy pedal on a disc brake swap |
Hey all, I did a search of the forum, and didn't quite find what I was looking for, so I figured I'd ask here. I've got a '63 Valiant with the full big bolt suspension and disc brakes from a '73 Dart Sport under it. When I did the swap, I made sure to order everything for a '73 Dart. I replumbed the entire car, got all new hoses, and a prop valve from Inline Tube. The master cylinder is a disc/drum, non power unit bolted directly to the firewall, I rebuilt all of the calipers, and wheel cylinders. There's no air in the system, I've bled all of the wheels, prop valve, master cylinder, etc. I'm running ATE 200 gold in the car, DOT 4.. not the Dot 5 silly-cone. The problem is that the pedal takes about 60% of it's travel to do anything. The car stops hard and fast after that, and the pedal doesn't pump up. It just goes most of the way to the floor before it works. I don't remember which master cylinder pushrod I'm using, but my guess would be the unit off of the '73 Dart based on the new part sheen it's got. I put a new master on it last weekend, because I've seen some reman-ed units do this sort of thing. The problem is the exact same. The pedal seems to have full throw, in that when it returns, the pushrod has maybe a 1/16" of play between it and the plunger in the master cylinder. I suspect the pushrod or the prop valve, but didn't want to drop the $60 on an adjustable pushrod if that wasn't the case. I also don't want to replumb the car so the discs get full pressure, and install a racing style valve in the rear circuit just to find out that wasn't it either. I'm currently working 60 hour weeks, so I don't want to spend a bunch of unnecessary time under the car. I'd rather drive it. Whaddya think? |
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| Author: | sandy in BC [ Fri Jun 02, 2017 3:36 pm ] |
| Post subject: | |
I think I would try to get less lash in your pushrod. You likely have a small dia M/C which gives good power with a long travel. |
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| Author: | DusterIdiot [ Fri Jun 02, 2017 5:41 pm ] |
| Post subject: | Is it? |
Quote: The master cylinder is a disc/drum
If this is new, did you bench bleed it? |
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| Author: | The Moffittman [ Fri Jun 02, 2017 7:30 pm ] |
| Post subject: | |
Yep, both master cylinders were bench bled before installation. How much should the pushrod be depressing the master cylinder at rest, if at all? I'd assume zero. I think I only have play if I physically pick up on the brake pedal. Would a larger diameter master cylinder have a firmer pedal at the top? That's what I prefer.. I'll keep power brakes in a car that came with them, but I don't care for the pedal feel. |
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| Author: | matv91 [ Sat Jun 03, 2017 12:50 pm ] |
| Post subject: | |
Just a guess,master cylinder ports reversed. I wonder if that makes a difference? |
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| Author: | The Moffittman [ Sat Jun 03, 2017 3:39 pm ] |
| Post subject: | |
The big reservoir is plumbed to the discs, the small to the drums.. and by that I mean that the rear reservoir (big) is plumbed to the front of the prop valve that has two outlets for each side of the front end. The front reservoir (small) is plumbed to the rear of the prop valve with the single outlet going to the back of the car. Triple checked that.. |
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| Author: | SlantSixDan [ Sat Jun 03, 2017 4:44 pm ] |
| Post subject: | |
What'd you use for a metering-proportioning valve/distribution block? |
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| Author: | matv91 [ Sat Jun 03, 2017 7:17 pm ] |
| Post subject: | |
[url][/url]"http://i1373.photobucket.com/albums/ag373/matv91/Front%20brake/Scan_Doc0001_zpskui8q2rw.png |
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| Author: | matv91 [ Sat Jun 03, 2017 7:19 pm ] |
| Post subject: | |
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| Author: | The Moffittman [ Sat Jun 03, 2017 7:20 pm ] |
| Post subject: | |
If you go to inlinetube.com or some hyphenated or underscored variation on that, you'll find a terrible web catalog. I got the stock replacement proportioning valve for a '73 Dart with discs. It looks exactly like a factory unit. I didn't want to replace everything in the brake system, and toss a junkyard part in the middle, so I dropped some coin on a fresh one. At the time, they were the only place I could find selling a fresh factory replacement. I'm sure there were other places to find one. After wading through countless Summit/Jegs universal or racing units, Inline Tube was the first site on a google search that didn't look like a total sham. Still, process of elimination points at the two final untested links in the chain.. the prop valve, and the pushrod. Like I said, I went digging for the one post where somebody had this problem, and couldn't quite find it. I'm not one to throw my money or time around willy-nilly, as I'm short on both recently. However, I'm sick of my flaccid brake pedal.. It's like stepping on an unexpected sponge. |
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| Author: | The Moffittman [ Sat Jun 03, 2017 7:29 pm ] |
| Post subject: | |
Matv91.. Jeebus.. I have a factory FSM for '73, and never checked it. Why didn't I think of that? I didn't think to check the bore on the new cylinder, but the old one is still on the bench. I'll measure it in the morning. Since they're acting exactly the same, and were ordered for the same application, it's a safe bet they carry the same diameter. Whatever, it's four nuts.. I'll check both of them. Thanks. |
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| Author: | matv91 [ Sat Jun 03, 2017 7:52 pm ] |
| Post subject: | |
Adjust rear brake shoes up, check wheel cylinder size and 73 may need residual valve |
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| Author: | matv91 [ Sun Jun 04, 2017 7:09 am ] |
| Post subject: | Master cylinder casting numbers |
http://therammaninc.com/files/categorie ... -19746.pdf |
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| Author: | SpaceFrank [ Sun Jun 04, 2017 1:18 pm ] |
| Post subject: | |
Quote: Adjust rear brake shoes up, check wheel cylinder size and 73 may need residual valve
What he said. Make sure you start out with the self adjusters extended far enough that you can just barely slide the drums on over the shoes. Also, make sure the self adjusters are working properly. I've noticed on some replacement drum brake parts kits that the cable isn't quite the right length for the lever to properly engage the wheel on the self adjuster.
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| Author: | SlantSixDan [ Sun Jun 04, 2017 1:21 pm ] |
| Post subject: | |
'73 was the first year without residual pressure valves (wheel cylinders redesigned to eliminate the need). Rear brake adjustment is a good thing to check. Speaking of rear brakes: what are you running back there, I'm assuming '73 10" drums? What bore size are your rear wheel cylinders, and what piston diameter are your front calipers? If it's all stock '73 stuff, you've got 2.6" front calipers and 15/16" rear cylinders, so after your spongy pedal is figured out there'll be some additional work eliminating the dangerous tendency of this setup to lock up the rears if you stare hard at the brake pedal. |
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