Braking problems

[Mike F]

Hi All
I've just finished fitting the linked brakes off a Midnight Special to my Martini and it's all bled through properly. But when I brake with the foot, it dips severely at the front.

I read a report on the Midnight and it said that the bike squats when braking. The differential valve at the rear of the master cylinder had marks on it that looked as if the previous owner had taken it apart.

Does anyone know how to set it up to how it should be?

Or does anyone have one spare that hasn't been taken apart

Keep revving

[Mike Hart]

Mike, it seems to me even if the proportioning valve were off, it shouldn't dip 'severely' especially if it didn't do that before with the old system of 2 calipers on the front mc, since now you are only grabbing on one front caliper w/ the foot brake. Hmm

when's the last time you checked/changed your fork oil?

[Mike F]

Mike
It's dipping the same as it did when it was twin front braking. That's why I'm thinking that it had been 'adjusted' to give more front braking. My wife's Moto Guzzi has the same type of system but doesn't dip when you use the foot brake.
The reason that I've changed the system is because I can't grip so good with my right hand - leads to veins breaking in my knuckles (it's heridary) and so want to get this braking sorted once and for all.

[Mike Hart]

As long as it doesn't lock up too easy, sounds like you've got it so you don't have to use the hand brake! At least the diving isn't worse, eh?

Progressive springs sure help the front end.

[Mike F]

It's the dipping that is worrying me. That means the majority of braking pressure is going to the front and not the rear. Under light braking, the proprtioning valve gives equal pressure to both and under severe braking, it gives more to the front. My problem is that it's giving more to the front hence the dipping under light braking. I daren't try it with hard braking yet.

[Jerry]

Man, I wish I had more time to reply to these threads...

On the XJ, with a rear proportioning valve, the pressure ratio between front/back varies. (This from memory, need to get home and grab a manual! There is a chart/graph/explanation of the action of the valve.) At lower speeds, back brake gets a bit more of the pressure. At medium the balance is about the same, at heavy braking the front gets more pressure. Due to weight transfer, front brakes do much more of the braking (can take more brake pressure) without locking up. Weight is transfered off the rear tire, making it easier to lock up.

The XJ brakes are linked, back brake and front left caliper. Front right caliper is operated from the front MC and handlebar brake lever.

My XJ doesn't dive much on the front end, but I have 1 year old Progressive springs in it. Old springs showed more dive. I would look at a couple things:

Replace the front springs.
Change fork oil, perhaps going to a heavier weight (15w instead of 10W)
Clean/rebuild proportioning valve.
Make sure rear brake is working, i.e. piston is not sticking in bore and is bled properly.

Personally I like the balanced system; it seems to mimic my original habit of always applying the rear brake first, then the front brakes. Have not heard a lot about front end dive when using it, so suspect the cause is with your bike and not a general characteristic of the linked system.

If you kept the original front end MC, remember it is now pushing 1 brake instead of 2, so the 'feel' will be different.

Just out of curiosity....did you swap in a different rear MC that had the valve with it, or did you install just the proportioning valve and associated plumbing? The linked system has a different rear MC than the unlinked system, larger bore, I think, to move more fluid.

[Mike F]

Hi Jerry
I know what you mean about not getting enough time on the PC, I was sat at mine 5.30 this morning just before I went to work.

Anyway with the bike, I've seen the manual on it and short of hooking it up to pressure gauges, I don't know how to test it. The manual says that if it's suspect - replace it.

I fitted progressive springs a year ago and so it can't be that and have 15 weight oil 'cos of the fairing. The MC that came with the progressive valve is a 3/4" off a Midnight Special - the original on my bike was 1/2".

I've fitted Stainless pipes throughout with Speedbleeders - I did have trouble bleeding the rear to front pipes until I replaced it with 1 hose dirct to the front caliper. There isn't any sponginess - it's rock solid.

I must admit that I haven't stripped the proportioning valve down - I was afraid that I might upset the settings or isn't there anything that can be upset. I haven't come across any exploded diagrams of it - anyone out there got one?

Has anyone ever stripped one of these valves down?

Keep revving

[Mike Hart]

Mike, I've never had mine apart (proportioning valve), but it's a fairly simple spring & plunger arrangement that as brake fluid pressure increases, the plunger moves to allow greater pressure in the front than the rear.

The service manual says that "A damaged or malfunctioning proportioning valve may cause overly sensitive rear braking." This would mean the plunger is not moving to 'proportion' the flow of fluid pressure to f/r calipers.

With brake fluid pressure is under (or at) 1961kPa (285psi), pressure is approx equal in front/rear. There is a spring loaded plunger that begins to move as pressure increases over that number, allowing more and more pressure to go to front, less to rear.

Sounds like failure mode isn't what you have. I suppose there is an outside chance that the plunger could stick in the 'high pressure' mode if it had no spring or very weak spring. Yikes.

The drawings in the manual aren't very detailed, but there are some seals in there too, which if faulty would result in too much rear brake (plunger won't move).

Hope that helps. (It helped me! I was wondering lately how that thing worked. )

[Mike F]

Mike
I think I'll take it apart because if it's only a spring and plunger there's not a lot to go wrong. There may be an adjusting nut to tension the spring to right pressure but if I count the number of turns to undo it, I should be able to reverse the procedure. When I've got it to bits, I'll take some photos so everyone will be that little bit wiser.
Thanks for the help and keep revving