Seca 750 brake issues

I have a 1981 seaca 750. Bought the bike and it had been sitting for a long time. Got it running and fixed electrical gremlins galore. Anyways the brake system was empty when I bought the bike. I filled it with fluid, bleed it out. But my calipers wouldn’t release. And would just stick closed on the rotors. So I took apart the master, cleaned the pressure relieve hole out, it was full of crap. Cleaned everything else out. It had a nice stream of brake cleaner going though it after no issues. Put it back on the bike and it wouldn’t build pressure. I disconnected the anti dives and it magically got pressure to both calipers. I bleed them out no issues. But same thing happens, the calipers are sticking closed. But not as bad as before. I’m at least able to move the bike by hand. Any ideas? Caliper rebuild maybe?

 

Whole system rebuild, including new brake lines.

 

The pistons in the calipers get pushed further out as the pads wear. This exposed area gets crud on it preventing the seals from releasing the pistons back. As K-moe said rebuild the whole system

 

I already have the seal kit for the calipers. Didn’t think I’d have to rebuild the master. Since the seals and everything were good and clean. How hard would it be to switch to a bar mount? And do I need to hook up the anti dives? Also where would i get new lines? They blow through cleanly no issue and aren’t worn or bad.

 

With old rubber lines, you can pinch the line between your fingers and thumb, apply the brake and you can actually feel the hose expanding.
This is why they feel spongy no matter how much bleeding you do. SS lines will firm this up.
K-Moe is the expert here on the idiosyncrasies of the 750 Seca brakes, I'm sure he'll recommend a kit from Chacal as that will likely supply the additional lines for the anti-dive. For sure if you're contemplating a bar mounted master now is the time to do it, just let Chacal know what you're doing.
The 750Seca probably has the most bizarre front brake system of any bike. Trying to buy SS Chinese lines from ebay for this bike is a guaranteed cluster-F.

 

I bought new braided steel lines from xj4ever on my seca 750 great upgrade . The hoses on bike were from 82 so they had to be replaced. I rebuilt both calipers and rebuilt both forks when I did mine . Stay away from a universal braided steel kit ....it may say DOT ..but probably is not fully compliant. The only thing on the seca is you have 4 brake lines I think the 2 long lines were 47 ea and 2 anti dive were 40 ea but don't recall you need to look at the catalog. I know sounds like a lot of money but how much is your safety worth? Cheers

 

Ok and thank you everyone. And the lever isn’t spongy. It’s firm. Just the calipers aren’t releasing. I’m gonna redo the seals in the calipers. And I’ll see if it helps. If not I’ll replace lines and rebuild master.

 

Rubber parts of the era had a 4 year service life.
Brake fluid is hygroscopic, and left unchanged will cause corrosion.

Why you need to rebuild your caliper and M/C w/pics

Why You NEED TO REPLACE Original brake lines w/pics

I don't even know you, and I think you're life is worth enough to spend the money on rebuilding the whole system.

 

Consider doing everything at the same time if you can. You could end up replacing seals in the calipers and then have something else dislodge from your "yet to be rebuilt master cylinder (or somewhere else down the line)," causing you more/different problems. There's no doubt more funky stuff in the system that you haven't been able to flush out yet.

 

Alright i will read into rebuilding the master and calipers. And source some new lines. I appreciate all the feed back on here. Thank you all!