Issues with rear caliper, not enough pressue?

[taron32]

All,

I'm hoping you can all make me feel stupid with some easy answers to my question. I'm working on a custom build of a 1992 BMW K75S (ABS). I'm almost done, with the front and rear brakes being the final items on the list. I just rebuilt the rear caliper which was fairly easy. I've installed some aftermarket rear sets from Powerbrick-Parts. I've also installed a Hydraulic brake light indicator for the rear master. Essentially once there is enough pressure it should close the circuit. Using an Ohm meter, I tried to test if the circuit was closing but it is not... i'm not sure if I am reaching that amount of pressure needed or if the unit is faulty. but this would make now two different units that have not worked which makes me think IM the problem.


Questions:
1. at the beginning of https://streamable.com/e/0jktgm this video there is a hole on the differential which I believe was for the ABS. When you delete the ABS what can i use to cover this up? I also remember there being fluid in here? Sorry if this is a dumb question...
2. in my same video you see me bleed the brakes to some very tiny bubbles. How long does it take to get rid of these bubbles? I've pressed the brake pedal nearly 1000 times over the past few days to build pressure and bleed and there are always a few stragglers. especially after letting it sit for a few hours
3. this might answer my hydraulic switch question as at the end of the video I am pressing the brake pedal down but I'm still able to spin the rotor with moderate resistance


Thank you in advance!

[Laitch]

The hole was for the speed sensor that communicated with the OEM instrument cluster's speedometer by means of a wire that attached to a plug hanging beneath the rear of the fuel tank. That's rear drive fluid you're seeing down in there.

[Martin]

The reservoir for the brake fluid is under the RH side cover.
Regards Martin.

[Laitch]

The fluid reservoir must be kept topped up during this operation. Failure to do that causes air incursion. Is the hose inside the bottle submerged in brake fluid? Tell us more about this gizmo you've installed, how it operates and how it was installed.

[rbm]

When you rebuilt the rear calliper, did you need to split the halves apart, or was it a simple refurbishment of the pistons?  Did you replace all the O-rings?

If you're not feeling the pads grasp the rear disk, then there is no pressure building up enough to actuate your switch.  I'm feeling the calliper rebuild has caused integrity problems with the hydraulics.

[taron32]

When you rebuilt the rear calliper, did you need to split the halves apart, or was it a simple refurbishment of the pistons?  Did you replace all the O-rings?

If you're not feeling the pads grasp the rear disk, then there is no pressure building up enough to actuate your switch.  I'm feeling the calliper rebuild has caused integrity problems with the hydraulics.

I took the halves apart and replaced the o rings. i followed the instructions on the kit and videos closely. Before this, the pistons were stuck inside the caliper and would not come out at all. but now they at least grip to a certain extent

[taron32]

I'll post a link to a videos below;

This video shows the setup of the new rear set with integrated master cylinder that i always keep topped off every time i bleed the fluid
"https://streamable.com/e/6e446d"

this second video shows me pressing the pedal, holding it, opening the bleeder for a few seconds (you can see some bubbles again as always) then closing it again before releasing the pedal.
"https://streamable.com/e/6e446d"

[Martin]

We need pictures of the master cylinder and the linkage set up. We also need the cylinder bore size and the length of the piston stroke.
Regards Martin.

[The Mighty Gryphon]

You have rebuilt the caliper, but have you done anything to service the master cylinder?  My experience(as well as others) with Motobrick rear master cylinders indicates that they are the weak point in the rear brake system. 

If the system isn't flushed every year water collects in the master cylinder and corrodes the bore making it possible for the piston to happily go back and forth in it without moving any fluid to the caliper.

There is a thread here that describes how to easily replace the expensive piece of Magura crap with a much better performing Chicom unit in two hours for less than $15.

[Laitch]

Have we at least succeeded in fulfilling your request for making you look stupid, taron? Some of us here need affirmation and we're willing to step up if we need to do more to get it. 

[taron32]

Have we at least succeeded in fulfilling your request for making you look stupid, taron? Some of us here need affirmation and we're willing to step up if we need to do more to get it. 

You’re getting there, but I thinkthere’s still room. I don’t hate myself just yet! I’ll send the requested info in a few hours when I’m back home

[Laitch]

I don’t hate myself just yet!

When you bring hate into the equation, that's a goal many of us won't attempt, and you're being too hard on yourself, also. Let's stick with stupid; that's achievable for any of us. 

[taron32]

haha never thought i'd thank someone for being called stupid but here we are. thank you.

So it travels about 13mm. I'm working with woodcraft right now. they recommended bleeding at the caliper banjo and master banjo with the pedal depressed to get the air out. Also, sent the video over to powerbrick to see if they have any thoughts on it. Will report back later today with the latest findings

I completely gutted and threw away half of the electrical components to install an M-unit and I'm being stumped by brakes!

[Laitch]

Judging from the latest image, you won't be helping the Chicoms with world domination just yet.  Which half of the electrical components did you discard?

[Laitch]

haha never thought i'd thank someone for being called stupid but here we are.

Feeling stupid is your goal, taron, one that you can fulfill for yourself at any time. We're just here to help members achieve goals, whichever they are.    Beyond that, what is the bore diameter of that brake cylinder? Martin and some of the rest of us are curious. I'm hopeful about success here with at least the brake situation.

[Martin]

Taron disconnect the line at the caliper. While keeping  the master cylinder filled up pump the cylinder until you get fluid out of the pipe. Keep the pipe elevated to help expel air. Place a thumb or finger over the open end to seal it and slowly apply the brake pedal to see if you have pressure. Reconnect the pipe to the caliper and bleed. This type of master cylinder holds very little fluid and can be harder to keep air from entering the the system.
Regards Martin.

[taron32]

i thought i updated but i guess i didnt! for anyone who used this just know that it takes forever to get all the air out! just keep at it

[beemuker]

There is a thread here that describes how to easily replace the expensive piece of Magura crap with a much better performing Chicom unit in two hours for less than $15.

It only took me an hour, as others have it completely figured out [with pics even]. ,but it cost me $20 as I used amazon prime [ I was in a hurry]

[The Mighty Gryphon]

Holy Mackerel!  June 30, 2022 to November 30, 2023...  It sure did take a long time to get the air out.  You certainly have a lot more patience than I.