Brake problem...........

[ROD]

Right,pay attention at the back please............All ideas/thoughts appreciated........
   I have Golf rear brakes and gsxr twin front discs,all powered from one 3/4" bore (I think) mastercyclinder.Originally the brake pedal travel was fine,but of late ,(ie after about 1500 miles max,I have driven the trike) the pedal has developed a lot of travel . It will just about stop the trike on one push of the pedal,but ideally needs to be released and pushed again,to give much better braking. Now,ordinarily I would put this symptom down to the brakes needing adjustment,but of course,being discs and pads ,there is no adjustment.I suppose they could need bleeding,but they dont feel spongy,just low. I havent looked at the pads,but Im guessing that they cant have worn down that far in 1500 or so miles? If they had worn down,or 'bedded in' would they increase the pedal travel that much? How do cars with discs all round compensate pedal travel for the pads getting thinner? I could lengthen the pedal to m/cyl shaft? I dont see any leaks anywhere else in the system.
  Also...I have problem with leakage (oo er) from the cap of the m/cyl. At first I thought it was leaking from a split in the screw top,but Ive sealed that up,and the fluid appears to be coming out of the tiny pinhole in the centre of the cap. There should be a disc of plastic in the cap that acts as a baffle to,I presume ,allow air in,but prevent the fluid escaping as it sloshes around.I doubt if this problem is associated with the low brake problem.The fluid does need a small top up occasionaly,but has never got so low that air could get in the system through the m/cyl.

[Olds]

The pads are pushed away from the disc by a very minute amount. A disc that is warped or out of true can cause a very small amount of excess travel but I think you would notice that.
Is the master cylinder a good make. Some cheaper ones have very soft bores and are prone to very fast wear, but 1500 miles seems low even for that. Unless the pushrod is not pushing in line with the bore which will wear the seals and bore quicker.
Is the pedal return spring good,  pulling the piston fully back. only when the piston is back or very nearly so, does fluid from the reservoir enter the cylinder to replace any that has leaked back past the seals, or to make up for wear.
It may be that the brakes just need bleeding.
Bikes and trikes tend to bounce around a lot more than cars. Bike reservoirs have a rubber bellows to allow for level drop without risk of leakage.
Might be worth looking to see if you can find one that will fit, or be modified to do so,  if you are worried about it.
Hope you can sort this one out.

[ROD]

Bit of an update........
 A thought Ive just had...Maybe when I first fitted the m/cyl it was already on the limit in bore size to push the caliper cylinders? and a small amount of pad wear has shown up the lack of cylinder bore size? Not sure what the next size up is. Im pretty sure mine is .6-- something or other
 
 Extremely  good points there ON! Just checked my pedal,and it snaps open when released,so thats ok. I seem to remember some old cars had a taller than usual m/cyl body extension,so maybe I'll make an extension on the m/cyl body to prevent fluid sloshing out.If I only top up the fluid to the existing level,that may do the trick to prevent the fluid loss.(I often wondered why bike cylinders were sealed top! ) Gonna have a go at bleeding the system tamara.

[Olds]

If yours is .6 something then its .625" next size up is .750" (3/4")
I use .625" just for the rear drums and I think I could operate those + one front disc from it.
Not sure how much fluid movement the golf discs require.
Rather than a reservoir extension, you could possibly fit a breather tube.

[spanners]

youre calipers ,,should,, auto-adjust, if set up correctly ,if the h/brake cable is set up wrong they won,t adjust ,

[scannerzer]

do you have residual pressure valves fitted

[tbone]

Or it could be that old chestnut.....volume vs pressure.

[hunter]

I think you hit it on the nose with it being on the limit rod,
Slight pad wear things loosen up,
Rod if you have flexi front brake pipes clamp on off,
And see if the pressure comes back,if so you may want to fit a 7/8 Master.

[ROD]

I do have a balance valve on the rear brakes,which I have recently slackened off a bit. I didnt mention that in my orig post as I was still getting the symptoms ,although to a lesser degree,before I altered the balance pressure. Cant clamp off the fronts easy,as they are stainless hoses.
  Ive already been rummaging through my boxes to find bits to make up a breather for the top of the cylinder. I found some stainless brake hose fittings,and I figure I will make up a elbow from a couple of fittings welded at 90 degrees,bolted through the centre of the cap,then run a piece of ,stainless flexy about 6 inches long upwards. that should stop the leaks from the sloshing about.
  I'll look in to the self adjustment of the rear calipers.
   I used the pads that came with the front calipers,and I dont know how thick brand  new ones are,so cant compare pad wear yet.
 I'll also get hold of a larger diam bore cylinder this week,as it cant hurt to push more pressure.

[spanners]

larger bore will give less pressure,,,,, but,,, more volume,,,

[ROD]

Got my volume/pressure mixed up there! Meant more volume,and more pressure to push the pedal  (I think!) Getting confused,its been a long day !
 
 Oh ,and hello T bone,long time no speak! 

[ByzMax]

do you have residual pressure valves fitted

This maybe what is required. Is the master cylinder mounted lower than the callipers? Fluid can run back into the master cylinder. The vaccuum can cause the calliper pistons to be drawn back a small amount creating the problem you have. The 1500 miles may be just that everything is moving freely now.

[ROD]

 
 Hmmm...Ive just read that res valves should not be put in a disc brake system ,as they hold the pads onto the discs?

[Manky Monkey]

2 lb valves for disc brakes.
10 lb valves for drum brakes, (fitted to my truck -discs on the front & drums on the back).

[ROD]

Yep,I understand now! The lower poundage ones are for discs. Can I just get away with putting one into the line from m/cyl to all the brakes,or do I need one in each line? (ie lots of pipework changes)