Front disc conversion: can't lock the wheels?

[theseoldcars]

Evening all,

Apologies for the slightly finger-in-the-air approach to this, but I thought I'd ask in case anyone else had experienced the same issue.

Basically, I fitted Summit's front disc conversion, which appears to be a reboxed SSBC kit. It's all fitted, done, but it won't lock the front wheels - and the braking effort feels very weak from the front. Need to stop in a hurry and, well, you won't. I get the distinct feeling it shouldn't be this bad, and because I would like others to drive my car (and I like to drive it hard) - well, I'm not happy with it.

Alas, I've read the thread about the proportioning valve - and unfortunately it doesn't make any difference, in my case. The rear brakes do lock very easily, however, and one drum appears to be continually binding up - which is annoying (the offside, nearest the brake line T).

This has led me to wonder if there was a mistake in the instructions, or my assembly, with regards to the ports on the master cylinder. The Summit-branded manual said one thing, the OEM manual said another. The lines I acquired brought the fittings up to correlate with the valving in the OEM manual, so I just ran with it.

Because the thing won't stop, however, and seems to like locking the rears - despite the valve - I'm wondering if they're the wrong way around? Could be I've got the wrong lines, somehow (I think they're a four-piece kit when mine should have been five, or something), so they've come up in the wrong place.

Any input appreciated. Know it's a bit unanswerable over the web, but just wondered if anyone else had experienced the same - before I start tearing through it. I'm still running the original distribution block, if that makes any difference - and this was an unassisted drum/drum car when new.

Here's the master as it stands:



Here's a bit of fun for you, too. When I got the car, I was told it 'stopped dead straight' (when it was on the road a decade earlier). As soon as I tore into it, I found it had a 10" drum the OSF, and an 11" HD drum on the NSF. Both were shot, though, so who knows!

Thanks all!

[theseoldcars]

(Alternatively, please shout if I'm expecting too much - but I would have thought that with 4-piston calipers, decent discs, modern pad materials and vacuum assistance, I'd be able to generate more stopping power...)

[Mopar Nut]

Have you checked the booster rod adj. going into the master?

[theseoldcars]

Not specifically; I followed the instructions and remember trimming a threaded bar down then adjusting nuts on it to get it all sorted on the pedal side... but outside of that, no. This was about a year ago now (eesh), so it's some way back.

Is there a sensible process to follow, with regards to that? May well have done it, so it might spark a memory.

Pedal travel seems okay, though. Part of me wishes (incorrect drum issue aside) that I'd just rebuilt all the OEM bits, haha....

[Kern Dog]

The MC port nearest the firewall is for the FRONT brake line, the port nearest the radiator is for the REAR brake line.
Lose the stock distribution block. The cars with disc/drum need a real proportioning valve whether adjustable or not. The wheel cylinders in drum brakes require far less volume to operate and if there is NO proportioning in the system, the end with drums is going to lock first.

[70-500-SE-EXPORT]

I have the same issue with my 68 after a front disc swap. I talked to a guy with the same problem with a 69 and he said he traced it down to his booster. He said the boosters that come with most kits are smaller diameter than stock and have little assist. He said he bought a repro bendix booster from "mbm " brakes and his brakes were super strong and he could lock up the fronts. My next purchase is their master/booster setup, im hoping it resolves my   sucky brakes. http://www.mbmbrakeboosters.com/index.php?option=com_virtuemart&page=shop.browse&category_id=17&Itemid=6

[Windsor]

Get a good perportioning block with a residual pressure valve. The residual pressure valve keeps a small amount of pressure in the brake lines. If you use inline valves, put them as close to the master cylinder as possible to keep as much of the line pressurized as you can. This will improve brake feel and prevent the calipers from "walking" the pistons back into their bores.

[Windsor]

I had te same issue as you when I did my disc brake conversion on my OE 1940 spindles. It would be loose in the front and drag the ass. Put in a residual pressure valve and then it would easily do a nose dive if I stomped it. The. Upgraded to a new master/booster/perportioning valve and it works great.

[440]

Is that a GM 1" master cylinder?

[hawkeye]

Where is your brake booster getting vacuum?  You appear to have it capped.   

[Highbanked Hauler]

 When I did the disc brake upgrade on mine I did a dumber than dog crap  move with the calipers. I put them on with the bleeder  pointing TOWARDS THE BACK  instead of POINTING  UP where it should be and couldn't get all the air out of it which caused similar problems to what you describe. I didn't put a booster in mine and it stops but I still can't lock up the rear wheels..