Brakes not building pressure

[GabrielGoes]

Good day mates, just wondering what yall think the culprit is the master cylinder or the calipers for my issue here on 80's G model not building pressure on the lever.

I completely disassembled my calipers, took out the o rings, cleaned the bore, and re-used the piston seal- they looked good.

I also cleaned the hoses and positively confirmed they are not blocked. i also checked the T fitting/junction where the 3 brake lines meet.

The problem is the lever never builds pressure. If I pump them up really fast I can feel a slight resistance. If I use a pair of vice grips to either side of the left or right hoses there is no difference... I already took my calipers apart and was about to order a caliper rebuild kit but I am thinking I should probably try switching my master cylinder.


For an issue with no pressure building up is it typically the master cylinder that is issue? I can't imagine it being the calipers because if the piston was seized still somehow, I would imagine at least the lever would get stiff.

Thanks for insight I just need to clear my head this is day 2 with these brakes am getting frustrated..

 

[bonanzadave]

Tell us how you bled the brakes after the rebuild.....

 

[GabrielGoes]

Tell us how you bled the brakes after the rebuild.....

With a vacuum pump. And also traditionally by hand. There's no air in the lines.

I'm going to end up in the garage again probably covered In brake fluid soon anyways just wanted to take a break and see what you guys thought. Master is an original I guess is probably the culprit. I really just hate brake jobs

 

[alke46]

Did you make sure that tiny hole in the m/c was clear and did you poke a very small wire through it to be sure? It has something to do with the returning fluid to the m/c (I think). But anyway, there are two holes in the master. One is a very tiny hole as opposed to the slightly larger one.

Hope this helps.

 

[GabrielGoes]

Did you make sure that tiny hole in the m/c was clear and did you poke a very small wire through it to be sure? It has something to do with the returning fluid to the m/c (I think). But anyway, there are two holes in the master. One is a very tiny hole as opposed to the slightly larger one.

Hope this helps.

I have not done this and you bring up an excellent point, I will check both holes. I haven't rebuilt too many m/c and I don't understand them fully. I might just throw the whole thing in the sonic tank for starters.

 

[alke46]

Clean the holes for starters, I think you will like the results.

 

[KiwiAlfa156]

Were the seals assembled dry? The piston has to be able to slip through the seal to self-adjust for pad wear. I had the same problem with new seals after a caliper rebuild and lubing the seal with a smear of rubber grease on the face of the seal solved the problem. The seal was distorting under hydraulic pressure -which its supposed to do as this is the 'spring that pulls the piston back off the pads- but because the piston couldn't slip past the seal it could never take up the excessive clearance between the pads and the disc. Pulled my hair out for weeks before finding the solution.

 

[Gorminrider]

Judging by your description, the problem IS air.

My most common problem when this occurs is a bubble in the MC...indeed the tiny hole in the Master is involved in my solution.

Firstly, protect the MC area and your painted surfaces -the gas tank! with clean rags...remove the MC cover and then the little shield from the top of the tiny hole-(there usually is one a vigorous pull of the lever may squirt fluid out of the MC )... if the little hole is clear, you will see some "activity" when you pump the lever slowly-(no more than 3/4 of the way)...Now, .get a small syringe from the drugstore-you don't need the needle- and apply some suction as you squeeze the lever slowly-a bit of technique involved one hand to pull on the syringe as you hold it against the tiny hole. OR an elastic to hold the lever in the pulled position.. I expect you will get bubbles as you patiently repeat this a few times....

Helps:
Tap your brakelines to shake bubbles....

There might be a better way to apply suction than the small syringe but they are useful so I keep several around...you MUST wash them out immediately for re-use later. The needle itself? cut the sharp end off and it's a "micro-oiler"etc

 

[chuck hahn]

did you put the cup that goes against the spring in correctly? The cupped side goes toward the spring.

 

[Nessism]

Clean the hole as mentioned, and force some fluid into the pumping chamber of the master. Before doing that though, replace the brake lines. It's silly to continue to use 40 year old rubber parts like those.

 

[chuck hahn]

If the return ports plugged what generally happens is the master pumps up till the pads are locked to the rotor and wont pump up anymore. I dont think its a return port issue.

 

[GabrielGoes]

Gosh it took Me a long time between waiting for parts etc. I found my solution to this issue to be that my piston seals were installed in the incorrect orientation. The larger "mouth" end needed to be installed towards the caliper, like the tapered skinnier end of the seal to be installed towards the piston. I had no idea that this was even a thing. I also used a smear of grease on the seals themselves.

Thank you all! I have double checked every suggestion here, so it is possible it was a collaboration of everything and not just the seals, however my return port was clear

 

[Nessism]

Grease on the seals? Was it special brake system grease? It's BAD to use general purpose grease inside a brake system since it will contaminate the fluid.

 

[Suzukian]

You should lube the brake seals with brake fluid.

 

[Nessism]

There is a special grease for lubing brake calipers and seals, but a special order product...

https://www.amazon.com/Brembo-Caliper-Seal-Grease-Packet/dp/B01M3Y10WV

 

[bwringer]

I suppose this particular rider is happy now, but one other thing to mention for future reference is "bench bleeding"; there is often air trapped in the master cylinder that needs to be burped out.

On a motorcycle, this is easy to do by moving the handlebar so the MC is more or less level, removing the brake lever and brake line, holding a finger over the brake line hole (acting as sort of a fleshy one-way valve) and slowly pushing the piston ALL the way in using something blunt that won't damage the piston or bore. Go slow or you'll get a jet of brake fluid in the face. Obviously, use lots of paper towels and protect the paint and instruments.

You can also bench bleed like this by arranging a piece of tubing such that it leads from the brake line port back up to the reservoir. You can kind of screw the plastic tubing into the port with a little experimentation. Using a finger is messier but a little faster.

Sometimes when chasing that last stubborn air bubble it helps to crack the banjo bolt loose to allow air to escape; push all the way in, loosen the banjo (keeping an absorbent rag around the fitting to catch the brake fluid), then re-tighten before allowing the piston to return.

 

[GabrielGoes]

Back to square one, the master I originally used to build pressure didn't have a mirror perch. The pressure wasn't the greatest but it was definitely there. I switched to another one and had no luck.

Fast forward to today, I put a brand new (in the package) brembo master cylinder for a bmw 310r which i had laying around, on and it doesn't build ANY pressure whatsoever and there's no air in the lines. Is it possible the master piston is too small to push the fluid efficiently to the caliper pistons??

this is absolutely maddening. I ordered new caliper piston seals and new hoses on the way as well.

Edit: going to try bwringers bleeding method above for good measure

 

[Suzukian]

I had bad brake response on my '83 GS750ES once, and I found that the slider pins were dry and binding. Just taking the calipers off, cleaning the holes out with a brass wire brush, and copious amounts of high temperature brake great, and the bike stopped right on the dime again. This has happened 3 times on this bike, but at that time I drove it everyday for commuting for 7 years.

 

[GabrielGoes]

-Stainless lines (can breathe through them fine)
-literally brand new brembo master
-bench bled the new master as stated above method (nice tip)
-slide pins are greased!
-vacuum pumped the lines and also did it the traditional way to make sure there's no air.
-bled the master at the banjo fitting as well
-caliper pistons are shiny - no pitting and looked good last I checked...

Im certainly not a brake phenom, but this is definitely not my first rodeo either... but my goose is cooked right now. This is crazy

There is absolutely ZERO pressure on the lever at the moment. I have my caliper rebuild kits coming in soon, maybe the grease I used on the seals compromised them? Or maybe the seals are just shot in general.

I will give an update after I install the new caliper rebuild kit with new piston seals..

I also re readr everyones post and ACTUALLY went through it all again from a clean start, not just saying "I did that already"...

Any other tips much appreciated if anything comes to mind.. my money is on the piston seals being bad...

 

[GabrielGoes]

To my demise, I took one of the calipers off to move it around and check to see if that would bring any air bubbles, sure enough, the seal was leaking, I lifted the boot and saw it pooling. The other caliper the same. After disassembling them both while waiting on my new seals - I can confirm what the gentleman said earlier about grease not being a good idea, it left the most hideous residue behind that made it look all goopy and brown, I don't think that was the issue directly at hand but I will definitely never use grease again- going to use brake fluid like I always did.

I'm not out of the woods yet so I will follow up when I install the new seals!