Friday, October 01, 2021

I'm not a doctor...

Stock photo: syringe shown is for mineral oil. Organic cotton mask by Graf Lantz. Not surgically approved.
Before I had to work with hydraulic brakes a lot, I didn't really know how to get the air out of a syringe correctly. When I would get an injection from a doctor, I didn't watch the procedure closely. Never stop learning! I'm not a doctor, but I play one in the workshop.

Yesterday's hydraulic fluid surgery had me rebuilding the lever and caliper of a SRAM Guide RS brake. Not only was the lever piston stuck in the characteristic way, but the caliper pistons were stuck. Water gets into the brake system in various ways. Oxidation and corrosion can follow. The glycol-based fluid SRAM uses absorbs water, so it is distributed evenly throughout the system. That reduces the effects of undiluted water pooling in a low spot, but does decrease braking power steadily, as the percentage of water increases and the boiling point of the fluid gets lower.

At least it was a front brake.

On the Lefty fork, you have to remove the brake caliper to unbolt the wheel from the axle. You should remove the wheel when working with brake fluid, to avoid contaminating the rotor. I was removing the whole brake anyway, because it had to be taken apart.

You can't start a job until the parts arrive, so that put me a bit behind schedule. The customer had hoped for the bike that day. It had been hanging in the shop for about a week, but we needed to get parts. They're lucky that the parts were available. Then nothing went according to the basic printed instructions or cheerful YouTube video tutorials, because every piston that needed to come out was jammed tightly in. I had to reassemble the caliper and put a spacer in, wrap the whole thing in a thick cushion of rags, and blast it with compressed air. That brought out three of the four pistons with varying degrees of willingness, but left one of them stubbornly buried. I had to reconfigure the spacer to hold the other pistons back a bit and leave space for the one holdout to expand into when it was finally willing. This took several tries.

The lever piston was also not responding. Compressed air doesn't help there, because the pressure vents into the upper reservoir of the lever rather than going full force against the recalcitrant piston. In that case, you can just clamp an old spoke in the vise, pointing straight up, insert it into the little hole where the brake line was connected, and tap the lever body down with a rubber mallet to dislodge the old piston. Feel free to damage that. It's not going back in. Just don't hit the lever body very hard with anything, and certainly not a metal hammer. Also be careful not to score the inside of the cylinder with the spoke end. It's pretty well guided by the size of the hole it's fed through, but if you get angry or frisky when hammering it could bend and give you worse problems than you already had. A scored cylinder will not seal, even if it just looks like a scratch. Hydraulic systems are very unforgiving.

After the tedious process of disassembly, the caliper halves and lever body need to be cleaned and inspected. Then all the new parts need to be installed cleanly and without excessive force. With the caliper pistons in particular, you're working blind once you go to shove the piston in, so you take it on faith that the seal stayed in place. There's quite a bit of resistance, because the seals have to hold sufficient pressure to stop a rider and bike going hell-bent down a rough slope. They're squared off and fit into a squared-off recess in the caliper half, but in a worst case you might fold one over partway. Once it's mangled, it's done.

The component on the bench never seems to look exactly like the examples in the manual. You have to determine whether the difference makes a difference. Are these the right instructions for this version of a component that may have been manufactured for a couple of years with the same model name and superficial appearance, but actually have critical differences inside that are not made obvious by any marking you can readily see? In that case, you may have ordered the wrong parts kit as well. The differences this time were not enough to stop the job.

Once the caliper was back together I had to assemble the lever. The current parts kit includes things that this old lever didn't use, but they were trivial. Still, getting a lively new piston in was fiddlier than getting the stuck old one out. The return spring on it fought hard against the insertion of a washer and spring clip that hold it in its proper position so that the little push rod on the cam that the lever actuates can do its thing, and all the magic juice stays in. You're working in the narrow interior of the lever body, to try to cram the spring clip at least far enough that you can coax it the rest of the way by pressing it with some object that gets it to snap into its little recess and properly engage. Except that it doesn't really snap, it just sort of stops and you have to keep peering in there with a light that you keep blocking with your own face as you try to align the light beam and your sight line to sort of confirm that you're pretty sure you've got it. That sums up almost all work on the most modern bike crap.

After successfully reassembling the whole brake, line and all, it was time to fill and bleed it. That's another fussy procedure that drips caustic brake fluid all over the place. Because it was a complete fill, there was a lot of air to chase out. The first go-round did not end with a firm lever feel. Because it always feels good on the bleed block you use to hold the caliper pistons back, you have to completely reinstall everything and test it with pads on the rotor to see if you've really got it. If not, the wheel comes out, the pads come out, the bleed block goes back in, you refill the syringes, hook everything up, perform the ritual again, and then reassemble to check, cleaning carefully as you go so that the brake fluid doesn't eat the paint on the bike or ruin the pads.

SRAM's instructions say to be sure to clean the brake fluid off of the lever and caliper, in part because the fluid's tendency to eat paint will remove the snazzy logos. Seriously? You guys have been working with this fluid -- by choice -- for how many years and you haven't come up with a way to apply the logos that's immune to it? Way to innovate.

It was well after official closing time when I left. It's bad luck to put any of the tools away when you're doing a brake bleed or a tubeless tire job until you're absolutely sure that it's a winner. I left the bike on the stand and the bleed kit strewn across the bench until I return today and make sure that no little air ninjas sneaked out of a crevice in the caliper.


RANTWICK said...

I recently switched from the entry level hydros on my Fattie to mechanical disc brakes. I couldn't be happier to get away from all that stuff! I'm glad somebody has the patience!

cafiend said...

Patience comes at a price. Our local trail builder has been forging alliances all around the region. He told us how a major shop in Portland, Maine, is going to jack service rates in response to the higher demand for technical knowledge and precision. The financial threshold for bike ownership is going steadily higher in the land of the privileged. Shops are having to get bigger or disappear. When no one can afford to be big, nothing will be left.

As the mountain biking culture reasserts itself here in a bigger and more intrusive way, with built infrastructure, the bike business will have to change with it. I will be shoved aside by the technobabies of the new generation, who fully embrace that bikes are expensive and temporary. If you can't afford it, find something else to do. I'll probably be out of a job pretty quickly. A rising tide does not lift all boats. It sinks a lot of them.