Terrible Front Brakes

FloppaEnjoyer8067

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The brakes on my 81 xs650 are terrible. It's been a while since I rode the bike due to it + my Honda grom being a blast, so I don't remember precisely when they became bad. I swapped out my top rubber line for stainless (I didn't do bottom -- long story), put in MikesXS Pads (29-3620) link. I think the brakes were worse around this point. I measured my front brake rotor and found it out of spec, so I replaced it and properly bedded it. This was all ~1 year ago. I'm riding the bike now, and the front brake is terrible. The rear drum has vastly more stopping power than the front. I'm looking to address this as I want to use the bike to commute again.

Reading around online, it looks like the best option is to swap the front master cylinder to 11 or 12mm. Before I throw more parts at the problem, does anyone have suggestions on other options?
 
I have had a suddenly bad brake issue related to a little fork oil seepage or brake fluid seepage onto the pads or rotor. Start cheap and spend later.
 
Use a 4 pot and a 12mm m/cylinder with a 320mm disc
 

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The smaller bore MC will be a big help... and one continuous braided line to the caliper is the route I went. I also swapped the heavy stock rotor for an XS performance rotor (Mike's XS sells). My front brakes (with stock caliper and new pads) functions extremely well.

If your secondary line is original, may be worth replacing it...could be swelling and limiting the flow of fluid. You may also want to clean and rebuild your caliper as next steps before doing anything else.
 
I went to disassemble the caliper just to check everything I could and the slide pin was completely siezed. I had to hammer it out.

I’m waiting on a new seal/boot for the pin to come in, and then I’ll put it back together and see if it fixes it.
 
I went to disassemble the caliper just to check everything I could and the slide pin was completely siezed. I had to hammer it out.

I’m waiting on a new seal/boot for the pin to come in, and then I’ll put it back together and see if it fixes it.
That’s most likely a big part of your problem. If the pin was siezed the pads would only push on one side of the rotor. Pretty sure you’ll notice a big difference once fixed.
 
So can anyone point me in the right direction for a 12mm master cylinder for my ‘75 in the UK please?
 
So can anyone point me in the right direction for a 12mm master cylinder for my ‘75 in the UK please?
Pic of your front brakes kind sir? A euro two disc front wheel with two piston calipers is going to need a larger bore MC than a late model single piston, single caliper set up would need.
 
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Here's a chart from 3M of VintageBrake fame. I marked the stock 650 sizes ......

MC Chart.jpg


According to his recommendations for ratios, a 14mm would work well for the older 2 piston caliper, if you only had one. He makes no mention of a dual disc (w/ 4 pistons), but I'm sure a 14mm would be better than the stock size. And the nice thing is, that's the size that came stock on all the '77 and later bikes, paired with the later 38mm single piston caliper. Now, that wasn't a good match, and that brake greatly benefits from an 11mm MC. That puts the ratio right about at the beginning of the 12 to 14 :1 range he likes for a single piston caliper.
 
I buy used O.E.M. units as well. They are the Nissin brand and very good quality. Normally, all they require is a thorough cleaning and no rebuild parts, except for the screws that hold the top on. They are usually rusty. At first, I didn't like the increased lever travel that the 11mm unit gives, but I've grown used to it. So, I tried a 1/2" (12.9mm) unit. This is good for lever travel, having hardly any more than stock, and it does improve the "feel" slightly, but not as much as an 11mm one does. So, I only get 11mm ones now.
 
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