Brake bleeding on 1990 Paso 750

[Graemer57]

I got my Paso out the other day to take it for its annual MOT only to find the front brake lever coming all the way back to the bars. Decided to replace the fluid and bleed the brakes, but couldnt get any pressure back in the line. So I then invested in a vacuum bleeder to mkae it a bit easier as i was, in all honesty, making a bit of a cack handed job doing it all single handed. Sucked the fluid through the system (though not as effectively as i thought it would) keeping the master cylinder topped up as instructed and.... still hardly any pressure. The front brake will now grab the disk but the lever is still so far back towards the grip i wouldnt try riding the bike. Ok I know the lines are old but they seem intact, with no bulges. There are no fluid leaks anywhere I can see. apart form investing in new brake lines what am I doing wrong? any suggestions?

Graeme

ps am not impressed with the Brembo designer who mounted the bleed nipples on the caliper INBOARD of the lines making it about as difficult as possible to get a key on and turn them with the wheel in situ. Repeat after me: Form and function, form and function......

[paso750]

When was the last time the bike was used ?
Although original brake lines should be replaced after so many years they are not your current problem.
If you absolutely can`t get any pressure to build up you may have to invest in a rebuild kit for the brake master pump (that is if the cylinder inside is ok). Bleeding the front brakes is usually not that hard.

G.

PS: don`t touch the brake lever adjuster screw.
Old brake lines don`t necessarily have bulges. It`s possible they dissolve internally and in that case they can clog, you would have pressure but the brake wouldn`t grab or they would but then not unlock. I had that on one car.

[Graemer57]

Bike was last used about 6 months ago and the brakes, while not brilliant were perfectly acceptable. I have got a small amount of pressure but still feel that the lever is way too spongy and comes back almost to the bars. It hard not to make comparisons to 'modern' brakes, especially as I've just ridden to Poland and back from UK on my BMW R1200RT which has servos assisted Brembos, but even allowing for the older design etc I know they should be better (as they were better!) Am beginning t woder whether I shouldnt be more systematic and replace the brake lines, rebuild the master pump etc

[bmw851]

Before going down the M/C rebuild and brake line road, I'd suggest it is time to do a caliper seal kit, had similar problems, did everything except the calipers, talk about backward completion principle, new seal kit and problem fixed in an hour and a bit.

My party trick for bleeding is to remove the calipers wedge the pads apart and go from there, bit fiddly but works for me.

Cheers

Peter

[Mc tool]

Its possible that the rear seal on the pump is leaking , not the seal that goes round the front of the piston but the o-ring that seals the piston/cylinder at the open end of the cylinder. Seems that this seal will function correctly until you try to bleed the brakes ( after draining and changing the fluid ) and then it will allow air to be sucked in as fast as you can bleed . Some times you can get away with just cleaning the parts and re-assemble , thou while you have it this far apart you may as well fit new seals . I have found also that bubbles can "hide " in the bango at the brake pump and in the union where the brake light switch is . Slackening the bango bolt and the brake light switch can allow any trapped air to escape ( use rag to catch any expelled fluid )

[bmw851]

Onya Mctool, you just solved a riddle that had me perplexed from when I did the 17” wheel conversion.

Bled & bled the front brakes and had the same problem as Graemer57, and still had the odd air bubble appearing in the master cylinder, obviously thought it must have had some form of “leak” in the M/C so to speak somewhere, but had enough by then and just wanted it finished so it could be ridden, so on goes a secondhand Nissin M/C from a Honda Blackbird, (yes a Honda and I don't care it works for me) wallah no more air bubbles, but horror still nearly just as spongy, it would stop but with a lot of work.

The problem aside from the M/C turned out to be an accumulation of water and gunk that had built in behind the caliper pistons over several years, which no amount of bleeding was going to shift, $50 for 2 seal kits, a bit of simple work and fixed, could now do stoppies if I really wanted too, well actually tried to do one in the driveway and nearly fell off. (idiot, and at my age)

So if that O ring is the problem then it’s an obvious fix, but I’d still suggest going the whole hog, lines and doing the calipers as well, couldn’t hurt too much hopefully .

Just remember to crack the cap bolts holding the 2 caliper half’s together before taking them off the fork leg.................................

BTW the Nissin Master Cylinder is a lot easier to bleed, it doesn’t shoot brake fluid 6 ft into the air , only a slight exaggeration.

Cheers

Peter

[paso750]

the caliper seals are not the problem if there`s no pressure building up. They would only be if they leaked.

[bmw851]

the caliper seals are not the problem if there`s no pressure building up. They would only be if they leaked.

Your right G, it’s not the seals, it’s because of the water content, amongst other god knows what things, rubber mud? from the inside of the lines perishing with age?, (and as you know brake fluid is hydroscopic so should be swapped out at least once a year regardless ), otherwise it just won’t compress like hydraulic fluid is meant to, there is something about fluid dynamic’s in there somewhere which I have absolutely no understanding of , but do know it is part of the reason for that ‘orrible “spongy at the lever” feel, so my point is, if you’re going to pull them apart to clean out the rubbish behind the pistons may as well change ‘em and the O Rings while you’re there, have another look at pic’s in my “Being Nasty To My 750 Sport” post page 4 I think, and have a look at the inside of the caliper’s when I took them apart , and they had have at that stage literally a litre of fresh clean brake fluid pumped and flushed through them and still looked like that.

At that point, had already fitted the new master cylinder and braided lines, done McTools party trick with the brake light switch and obviously was still having problems, seal kit isn’t maybe the right term, as that infers they are the issue and in some cases well could be, but this to me sounds like a well maintained bike just suffering from a bit of old age plaque build up so to speak

edit: Sorry forgot to say try as McTool suggested the master cylinder seal and O ring kit 1st, lot less work and may fix it there and then, I'm just OC about brake calipers now, goes back to a '78 Honda 4 I restored .

Cheers

Peter

[paso750]

it’s not the seals, it’s because of the water content, amongst other god knows what things, rubber mud? from the inside of the lines perishing with age?, (and as you know brake fluid is hydroscopic so should be swapped out at least once a year regardless ), otherwise it just won’t compress like hydraulic fluid is meant to, there is something about fluid dynamic’s in there somewhere which I have absolutely no understanding of

hydroscopical means it attracts water. Water is considered not compressible. (it takes 200bar for 1% compression). So that does not reduce the brake pressure. The problem with water is that it lowers the boiling point of the brake fluid. If the water content is over 3% brake fluid needs to be replaced. As a rule of thumb brake fluid should be changed every 2 years.
It`s not necessary to do that every year (although I did that, too at the beginning of every season).
Brake fluids will hold up more than 4 years in a closed system. Digital brake fluid testers don`t cost much btw.
If you don`t get brake pressure it`s either bad seals or something compressible in the system (air or any kind of deposits while the latter often cause the opposite)

so my point is, if you’re going to pull them apart to clean out the rubbish behind the pistons may as well change ‘em and the O Rings while you’re there

agree with that although it`s not a must but I`d do that, too

[bmw851]

Thanks G, as said I don't profess to understand fluid dymanics, but had always thought aside from obviously air in a brake system the water content was the other culprit, plus any sludge behind the pistons of course .

So thanks for making that clearer

And I now can't explain it, you've shot my firm held theory on what was wrong with mine down in flames , take it as a given no air in the lines, cleaned out and rebuilt the calipers and just like that fixed.

So buggered if I know now.

Cheers

Peter

[Mc tool]

Ive "done " a few sets of calipers lately , mothballed the brembos off my SS and spare set for the Paso ,rekitted my Guzzi 850t ,my ford Capri and Di's MR2 and was amazed at the amount of crap that had accumulated inside the calipers in all of them. The cars have had the rubber hoses replaced ..... the paso is next , the Guzzi has the braided lines off the SS

[paso750]

while all my brake calipers were clean inside (I wonder if that is a result of regular fluid change) my rear brake pump was full of crystalline stuff. I still wonder what exactly that was

[higgy]

that is what caliper grease looks like after brake fluid has sucked all the water out of it