Rear Brake analysis, what is going wrong?

[Nobbsie]

So i have just got a new bike, has been sitting round for a few years.
The rear brake is super spongy so I came to the conclusion that it must be old fluid, water air in the line.
Have just bled the rear brake with new fluid.
The problem is still there.
you push the brake pedal once and there is nothing the but if you repeatedly pump them you get very minor braking,
I am unsure where to go from here, brake lines or seals?
I know it is losing pressure somewhere but it is not losing fluid.....
any tips on if it is more likely master or slave?
Cheers
Nobbsie

[caspernz]

Visible leaks?

In the absence of a visible leak, if you're losing pressure with the pedal depressed I'd look at the master cylinder, may be as simple as replacing perished seals.

[Nobbsie]

No visible leakage at all!
That was what i was thinking to might try pull it down tonight and have a look
Cheers

[Motu]

Seized slide pins.

[F5 Dave]

Old bike I like to pump out the pistons (pads out slowly pump both pistons evenly till they can be wriggled out by hand top up if required) pull out the seals clean the white powder behind the seals and inspect seals.

If in doubt new but main seals that are square usually last. Check sliding pins if fitted. Clean bore inspect for corrosion. Light scotchbrite or replacement same with pistons.

That should bleed up well after a long time expelling air.

If not then master cylinder but your time has not been wasted.

Some oddballs have to be upended to get the bleed nipple to the highest point but unlikely on a jap bike.

[Tazz]

As above pull the caliper off and check/clean. Caliper kits are what, $20?

Also check the line hasn't gone soft and is 'ballooning' out at some point when pressure is applied, but do the caliper first.

[Nobbsie]

Good as gold i might do that too
thanks so much for the info guys

[jellywrestler]

As above pull the caliper off and check/clean. Caliper kits are what, $20?

.

shit, i missed the bit where they said make and model. what about alignment, does the whole caliper move when the brakes are applied, part of your travel could be taken up with allowing for this movement. take the caliper off and appy it directly to the disc and see whether it works better.

[Nobbsie]

No caliper does not move,
Sorry it is a gsxr 400 1991

[jellywrestler]

No caliper does not move,
Sorry it is a gsxr 400 1991

Do the pads move inside the calipers?

[Akzle]

fuck me a helpful thread on KB.

you're lucky noo guy. fokken lucky.l

[FJRider]

The rear brake is super spongy so I came to the conclusion that it must be old fluid, water air in the line.
Have just bled the rear brake with new fluid.
The problem is still there.
you push the brake pedal once and there is nothing the but if you repeatedly pump them you get very minor braking,
I am unsure where to go from here, brake lines or seals?
I know it is losing pressure somewhere but it is not losing fluid.....
any tips on if it is more likely master or slave?
Cheers
Nobbsie

Sounds like air is getting in ... probably through the bleed nipple. (but not necessilary)

Check also the resovoir lid seal. (leaks often show below fluid level)

[sidecar bob]

Pistons will be stuck to the caliper seals & will be pulling the pistons back further than normal when the brake is released, requiring more fluid displacement to operate the brake.
Strip the calipers, clean out the seal grooves with a scriber wire buff the pistons & wash the whole lot, seals, dust boots calipers & all in warm soapy water. Reassemble with red rubber grease on seals & pistons, bleed up & all will be well.

[mossy1200]

Pistons will be stuck to the caliper seals & will be pulling the pistons back further than normal when the brake is released, requiring more fluid displacement to operate the brake.
Strip the calipers, clean out the seal grooves with a scriber wire buff the pistons & wash the whole lot, seals, dust boots calipers & all in warm soapy water. Reassemble with red rubber grease on seals & pistons, bleed up & all will be well.

This guy. 1+

[Tazz]

shit, i missed the bit where they said make and model.

Fair call, but a bunch of cars and 3 bike brands deep and I'm yet to come across exorbitant prices for some seals + whatever else it needs in the way of circlips and rubbers.

http://www.ebay.ie/itm/GSX-R-400-RL-GK76A-1990-Rear-Brake-Caliper-Full-Piston-Seal-Kit-/131352553210?pt=LH_DefaultDomain_3&hash=item1e9538e6fa

Looks like you can get a new piston and seals for $50< for this model, but the advice above should suffice.

[awayatc]

If your disc is warped it pushes your pads back out when wheel rotates.....
can be mistaken for need to bleed....

[mossy1200]

If your disc is warped it pushes your pads back out when wheel rotates.....
can be mistaken for need to bleed....

If its not a floater is there a lot of chance its warped? Maybe if its taken a knock.
After getting my wheels powder coated the discs were forcing pads away like this because of a small amount of paint on the mating surface. Discs slight out of line. Not fun on the front end when you need pump fast to stop

[jellywrestler]

If your disc is warped it pushes your pads back out when wheel rotates.....
can be mistaken for need to bleed....

hence the advice to try it unmounted and direct onto the disc to eliminate a few of the possibilities

[awayatc]

hence the advice to try it unmounted and direct onto the disc to eliminate a few of the possibilities

Not arguing....
just adding cent or 2....

[manxkiwi]

Someone else mentioned up ending the bike to get the calliper bleed nipple to the top. Just wondering if the brake hose does a bend over the swingarm? As in; the high point of the hose is above both the calliper and the master cyl? If so you can have a fair bubble of air sitting in there. No amount of bleeding will help as the bubble will always stay at the top, obviously. Try whipping the calliper off and bleeding it whilst holding it up so there's an obvious route for any air to make it to the bleed nipple.

My Triumph is like this and I have done all this before. Thought your set up might be similar?

Of course all the cleaning servicing type advice is worthwhile also.

[sidecar bob]

Someone else mentioned up ending the bike to get the calliper bleed nipple to the top.

You're fuckin shitting me, did someone actually suggest that? that's kinda like the old joke about how many Irishmen it takes to change a lightbulb, one to hold the lightbulb & three to turn the ladder. Wouldn't it be easier to dismount the caliper & turn that upside down? But hey, I'm no expert.

[nodrog]

You're fuckin shitting me, did someone actually suggest that? that's kinda like the old joke about how many Irishmen it takes to change a lightbulb, one to hold the lightbulb & three to turn the ladder. Wouldn't it be easier to dismount the caliper & turn that upside down? But hey, I'm no expert.

i think this guy was trying to bled his brakes that way

https://www.youtube.com/watch?v=TPHd_2RJHyU

[RideLife]

Another issue I've noticed a few times, is when someone has adjusted the lever movement to a point where the moving piston seal inside the master cylinder can not retract enough to expose the 'from the reservoir' intake hole. This means the master cylinder can't suck from the reservoir, and get enough fluid the presser the line for the next stroke.
While bleeding, you should be able to observe the reservoir fluid level go down, down, down. If it goes, down, up, down, up, While Bleeding.... that could be the problem.

[jellywrestler]

make sure you put a different colour brake fluid in each brake system, ie one colour in the front and a differrent in the rea, that way you'll be able to identify which brakes are leaking.

[Nobbsie]

Thanks for the advice guys!!
All sorted..... :Punk:
Didnt have to to pull the brakes apart all though will rebuild them next month when have available funds
All ready to be re vinnd next week
only thing that needs a good deal of work now is my riding :facepalm:

[Bassmatt]

I have a similar problem. What was the fix in the end?

[Nobbsie]

The fix for me was that im a idiot :facepalm:
I had bled everything in the wrong order :facepalm:
I bled the master from the banjo to make sure there was not air in it, then bled the caliper the right way round, and vuala all fixed.
i had bled it the wrong way meaning the air and crap was just getting pushed around and not getting out of the system.

[manxkiwi]

You're fuckin shitting me, did someone actually suggest that? that's kinda like the old joke about how many Irishmen it takes to change a lightbulb, one to hold the lightbulb & three to turn the ladder. Wouldn't it be easier to dismount the caliper & turn that upside down? But hey, I'm no expert.

No shit. Last comment of post #5.. It wasn't a suggestion as such, just mention that that had been heard of.

[F5 Dave]

Indeed, I've seen a number of Trials bikes with out of the way but poorly thought out rear callipers. And yes, lifting the rear up on a crate while leaned on the fence is the easiest way to do it without the calliper moving around, you only have so many hands.