Rear Brake Pedal Sinks When Releasing Front Brake

Returning from FerrySTOC my rear brake pedal would sink when I released my front brake lever. I read in the forum that water in the brake line caused this. OK, I road through a lot of rain, so I flushed the brake fluid. I saw no milky fluid, in fact it was down right clean. I thought the problem fixed.

I replaced the front brake pads since they were getting thin following FerrySTOC.

On the ride to RockSTOC the problem returned after a few hundred miles. Well I'm back now and I'm puzzled what could be causing this and want to get it fixed.

FYI I installed a new SMC assembly in Sep 2015. It's clean and works, I resealed it with brake grease when I flushed the brake fluid. Individually the brakes are firm, front and rear.

Any ideas?

Reginald said:

Returning from FerrySTOC my rear brake pedal would sink when I released my front brake lever. I read in the forum that water in the brake line caused this. OK, I road through a lot of rain, so I flushed the brake fluid. I saw no milky fluid, in fact it was down right clean. I thought the problem fixed.

I replaced the front brake pads since they were getting thin following FerrySTOC.

On the ride to RockSTOC the problem returned after a few hundred miles. Well I'm back now and I'm puzzled what could be causing this and want to get it fixed.

FYI I installed a new SMC assembly in Sep 2015. It's clean and works, I resealed it with brake grease when I flushed the brake fluid. Individually the brakes are firm, front and rear.

Any ideas?

Click to expand...

A good bleed will fix the pedal drop upon trailing off the front brake. Air in the proportioning control valve or the SMC. What happens is the front brake thru the SMC link applies the rear center piston against the rear disk but when the lever is released the outer rear pistons that are activated by the foot lever are not quite pressed against the disk due to the air in the circuit. The little extra travel of the "drop" pushes the outers against the disk.

dduelin said:

......the SMC link applies the rear center piston against the rear disk ........the outer rear pistons are activated by the foot lever .....

Click to expand...

Dave,

That's the opposite.

Pedal energizes the Center and SMC the Outers.

Well Ok. The mushy center piston causes the pedal drop.

Thanks. Guess I'll flush the rear linked system again.

You probably do this (remove and tilt) but if not the offending circuit goes through the SMC which has to be tilted. Those rear caliper circuits are very long through the delay valve, proportioning valve and SMC and it's easy to leave a bubble along the way.

Dave,

It may be interesting to follow up on the theory of the "Mushy Center Pistons"?

The run to the Rear Center is quite short and direct from the Rear Master to the Center. Rear Center mushiness would be caused by air in this section of the system.

So maybe start by purging this short rear section first and give it a try and see if it solves the Pedal Drop?

If purging the whole system in one go, we wouldn't know if the Drop is related to Rear Center or not.

What do you think?

My thoughts? I had pedal drop when I first got my bike and the first brake bleed I did did not remove it. I took a long trip with it like that and when I got home I slowly and deliberately bled it again to get rid of it. Ever since then I do a flush and bleed by the steps, circuit by circuit and removing and tilting the LF caliper after finishing the front outer pistons. The pedal drop has never returned. I never have done part of it or taken a short cut.

I posted earlier today from my phone during a ride and from memory. Now at home looking at the bleeding guide I use I see that you are correct. The rear center is very short. The longest circuits are the proportioning control valve and the front centers. Air can be difficult to purge from the PCV circuit and from the front centers and rear outer. These take 20 inches of vacuum and pumping the foot pedal to purge. I pump two or three full reservoirs thru the PCV circuit as often a bubble comes out late in this very long circuit.

Reginald said:

Any ideas?

Click to expand...

There are two rear master cylinders... one up front on the left front fork and one under the right foot brake.

Releasing the front caliper lever ( when moving ) allows the SMC to "relax" , taking away pressure and volume from the rear caliper .... which has to be replaced by the rear master cylinder..... petal sinks

This might suggest a PVC problem.

Air still in system...strap front lever back and place a weight on foot pedal and call me on the white courtesy phone tomorrow, I think I can help.

Interesting...I never noticed that pedal drop until I started riding the 2010. My first ABS bike. I just assumed it was normal after experiencing my brand new 2012 doing the same thing.

Guess I'll have to see what I can do about it!

Thanks, Guys!

Igofar said:

Air still in system...strap front lever back and place a weight on foot pedal and call me on the white courtesy phone tomorrow, I think I can help.

Click to expand...

I'll do this, but I need to do some major work around the house first, finishing doors and such. I've put it off far too long in favor of riding. I'll get back to the bike in a week from today.

I'll strap the lever and weight the pedal and see if this relieves some of the air. Next morning I'll have someone push in a tilted SMC (with strap wand weight still applied) and release the PVC bleed valve - close bleed valve - release the SMC and repeat a coupe of times. Then bleed normally. I'll only bleed from the rear master cylinder in the correct order, skipping the front brakes which are fine.

It's funny how this just showed up on the last day of riding home from FerrySTOC. It makes me suspect the PVC, but I'll go with the experience and bleed it all again.

Thanks everyone.

Don't just do the rear section only, start from the beginning and do all bleeders, as you don't know where the air is hiding.

Igofar said:

Don't just do the rear section only, start from the beginning and do all bleeders, as you don't know where the air is hiding.

Click to expand...

I would guess Reg means he'll leave the front brake circuit alone, which is fine?

While some would consider leaving the front brake circuit alone, this would not be the correct way to remove air from the brake system, as you don't know which circuit the air is trapped in.
One circuit can simply push air to a different area and conceal it.
The best way, is to follow the correct procedure and start from the beginning and do them all.
.02

Igofar said:

One circuit can simply push air to a different area and conceal it.

Click to expand...

I'm going to follow you're advice. But I'd like some clarification on my knowledge of the ST braking system.

I thought the front brake circuit was separate from the rear linked circuit. IE when the front brake lever is squeezed it presses the outer, front pistons in, and doesn't activate the rear linked circuit unless moving. Then (if the front wheel is spinning) the SMC is actuated pressing the outer, rear pistons; thus, one of the linkages between the two. The other linkage is when the rear pedal is pressed a brake line circuit is activated through a delay valve to the center pistons on the front calipers.

I've not had the front calipers apart so I can't say whether or not the front mixes the circuits, but it seems unlikely.

Thanks.

Reginald said:

I'm going to follow you're advice. But I'd like some clarification on my knowledge of the ST braking system.

I thought the front brake circuit was separate from the rear linked circuit. IE when the front brake lever is squeezed it presses the outer, front pistons in, and doesn't activate the rear linked circuit unless moving. Then (if the front wheel is spinning) the SMC is actuated pressing the outer, rear pistons; thus, one of the linkages between the two. The other linkage is when the rear pedal is pressed a brake line circuit is activated through a delay valve to the center pistons on the front calipers.

I've not had the front calipers apart so I can't say whether or not the front mixes the circuits, but it seems unlikely.

Thanks.

Click to expand...

Correct. There is no hydraulic connection between front and rear. No chance for air to migrate from one circuit to the other.
If the rear were to fail, you'd still be left with a completely independent front brake activating 4 pistons.
Bleeding the front just because you are doing the rear is like bleeding your wife's car just because you are bleeding yours. Just my opinion of course. But also based on perfectly good partial bleeds made on my ride.

On your description of the rear system. If you activate the pedal alone, front middle will also work to engage the smc if you brake hard enough to overcome the delay valve, and engage the rear outers, with far less force than if you had the front engaged though.

I have this pedal drop to, but I kind of ignore it as I can stop on a dime a get a nickel change with these brakes.
I know it's bad for the rotors, but I always use EBC brake pads for thier stopping power.

As for the front and rear hydraulic system, they are totally independent. But, you want to get all of the air and water out on an annual basis, just like you doing it for the rear hydraulic system.
Your secondary master cylinder (SMC) uses the rear master cylinder brake fluid to function also. There might be a bit of confusion on this as it is mounted on the left front fork.

For the new guys on here, head over to the article section and watch the video on how to bleed these brakes on a linked system.

https://www.st-owners.com/forums/showthread.php?68913-ST1300-Brake-Fluid-Replacement

Al Gully said:

I have this pedal drop to, but I kind of ignore it as I can stop on a dime a get a nickel change with these brakes.
I know it's bad for the rotors, but I always use EBC brake pads for thier stopping power.

As for the front and rear hydraulic system, they are totally independent. But, you want to get all of the air and water out on an annual basis, just like you doing it for the rear hydraulic system.
Your secondary master cylinder (SMC) uses the rear master cylinder brake fluid to function also. There might be a bit of confusion on this as it is mounted on the left front fork.

For the new guys on here, head over to the article section and watch the video on how to bleed these brakes on a linked system.

https://www.st-owners.com/forums/showthread.php?68913-ST1300-Brake-Fluid-Replacement

Click to expand...

If you want to you can get rid of the pedal drop and have brakes that operate as well as Honda intended. It's about feel on the controls as well as absolute stopping power. Once the drop is bled out and the system is systematically kept up to par the drop will not return. On the long trip I mentioned earlier I was on mountain roads trail braking into curves and realized the pedal drop wasn't normal and it was affecting feedback from the rear wheel as I trailed off the front brake while still trailing a bit of rear.

Explain how you were able to trail brake (applying rear brake only) with a linked brake system?