Brake pedal travel.

[Bramlett]

How much travel should there be on the pedals before the brakes are activated?

Our 170b's brakes are really sensivtive. The slightest pressure and they are on, either side. So how much travel is normal? Ive gotten somewhat used to them, but am curious what is normal for the design and what everyone else thinks.

Thanks,

Patrick
N2542C

[ginbug92b]

To gain a little pedal free play you could drain a little fluid out from the bleeder on the brake caliper. They could be over serviced, hard to tell without actually being there.

[c170b53]

How old are your brake calipers and when was the last time they were apart?

[Bramlett]

To gain a little pedal free play you could drain a little fluid out from the bleeder on the brake caliper. They could be over serviced, hard to tell without actually being there.

Thanks,

[Bramlett]

How old are your brake calipers and when was the last time they were apart?

I'm not sure. We just bought it a couple months ago, ill have to o through the logs. They are cleavelands i do know.

Thanks,

[Bramlett]

So how much travel do you all have in your pedals before the brakes activate?

[Bruce Fenstermacher]

This is a battle I've also been having. First some back ground. I had what was essentially a '52 B model. Brakes, brake pedal travel were never a problem. New to me 170A with Goodyear brakes and crosswind gear and older style master cylinders that by SB are supposed to be replaced.

Very touchy brakes. In fact for the first 3 months I flew it as an owner I told my partner who as owned the plane for 35 years, that the right brake was coming on with just pedal deflectionn no brake application. He insists nothing is wrong. I proved to him the brakes where coming on with just pedal deflection by spinning the wheel and applying rudder.

So what was wrong and what was applying the brake with just rudder pedal deflection. I got VERY familiar with the pedal, master cylinder geometry. But couldn't figure it out. Placed a call to Del Lehman cause two heads are better than one. We are talking about this break linkage and how it all works when he says they need to be adjusted 6" front the brake pedal hinge point to the firewall.

Sure enough mine were adjusted about 4 3/4" from the firewall when they were neutral. What this allowed to happen what that the geometry of all that linkage ran out of travel and the brake was applied just by moving the pedal. To adjust the pedal distance from the fire wall you need to go back to the rudder and shorten the rudder cable length pulling the pedals away from the firewall. In my case most of the problem is gone with just this one change.

After checking and adjusting the pedal distance to the firewall you should then look at the adjustment of the clevis at the top of the master cylinder. It can be raised or lowered which also changes the geometry of the linkage system. You are looking for the linkage to work freely and not apply an brake (downward force on the piston rod) no matter where the rudder pedal is place in it's travel.

If I understand correctly George is suggesting that the master cylinder might be over serviced and that level of fluid in the reservoir makes the master cylinder more sensitive or in effect work better. This is not the case with this design. As long as there is enough fluid to fill the cylinder below the piston the master cylinder will displace the same amount of fluid regardless of reservoir level.

We are assuming seals and adjustment of the Lock O Seal in the master cylinder are correct and functioning properly. About the only way I can see that a master cylinder would be more sensitive than another in the Lock O Seal adjustment. There should be .040" between the piston and the Lock O Seal. If this is tighter the piston placed higher on the piston rod and more fluid would be displaced and sooner in the piston travel. In other words more sensitive. But .040" isn't much and if it is to tight no fluid would travel from the reservoir into the compression cylinder and the whole thing wouldn't work at all.

BTW while studying the service manual to write this I note a warning to make sure the cap or plug of the reservoir is vented. This is a simple 1/16" hole in the plug. If your's isn't vented or the vent is plugged pressure could build the manual says. I might imagine this pressure might activate the brakes in some unwanted fashion.

[canav8]

Bramlett, take apart your master cylinder tops. Pull the pushrod out and check to see that you have .040 clearance between the lock oring and the end washer according to the 100 series service manual. These references are at the hangar so I apologize that I cant tell you to turn to a specific page, but if you have hair trigger brakes, you probably have have no clearance between the lock oring and the washer. I will go to the hangar tomorrow and get a reference but look up brake adjustment in the 100 series manual and you will find the tolerance. Adjust both and you will probably have trouble free brakes. I cant tell you how many times I have seen these mis adjusted by other mechanics. It is a common problem. D

[GAHorn]

...If I understand correctly George is suggesting that the master cylinder might be over serviced ....

Where did I say that???

[Bruce Fenstermacher]

...If I understand correctly George is suggesting that the master cylinder might be over serviced ....

Where did I say that???

To gain a little pedal free play you could drain a little fluid out from the bleeder on the brake caliper. They could be over serviced, hard to tell without actually being there.

WOOOOPs. Sorry George, Mark (ginbug92b) said that in a round about way. But his login, ginbug92b, does start with a "G". I guess we might be even on the St. Pauli Girl.

(And Mark, I'm sorry as well. )

If we were talking about as brake system that used the Scott diaphragm type master cylinder found in J-3 Cubs and many Pipers after it, then Mark would be correct. On these master cylinders the "reservoir" is closed and the diaphragm pressurizes all the fluid in it. Less fluid in the system means more air is present to be compressed before fluid moves down the brake line to the caliper and so you would have more brake pedal travel.

[Bramlett]

Bruce and Doug,

Thank you very much. This gives me at least a starting point!

I think that I have a 100 series service manual in the box of goodies that came with the plane.

Regards,

Patrick

[Bruce Fenstermacher]

To gain a little pedal free play you could drain a little fluid out from the bleeder on the brake caliper. They could be over serviced, hard to tell without actually being there.

I just thought of another scenario in our 170 system where Mark's comments about bleeding the brake caliper would give more brake travel. And it is not because the brake system is over serviced. It's because the Lock O Seal in the master cylinder is not opening.

How the Lock O Seal works is at rest it would be open releasing any pressure in the brake caliper, brake line and master cylinder compression area. At rest fluid is free to flow from the system up into the reservoir and vice versa. If the Lock O Seal did not open then you would in effect have a closed system with not reservoir. Your brakes would work fin as long as there was enough fluid displaced by the master cylinder to open the caliper and no air got in the system. In this case if the system between the caliper, brake fluid line and master cylinder pump came under increased pressure, then touching the brake pedal would immediately apply brakes. And even if it didn't come under increased pressure the free play of of the opening and closing of the Lock O Seal would be lost in the system.

In this case releasing some brake fluid from the caliper would lower the brake pedal and depending how far open the calipers are pushed, add some free play, but your not fixing the root cause of the problem.

To test for a closed Lock O Seal I'd try adding fluid to the brake caliper and watch the brake pedal. If the brake pedal top moves aft when fluid is pumped it then the Lock O Seal isn't opening and you will build pressure in the brake line which shouldn't ever happen with the master cylinder at rest. This would only test the Lock O Seal is opening, not that it is adjusted for the proper opening.

Below is a picture of the master cylinder in which I've colored in the area brake fluid would be.. Looking at the blow up of the Lock O Seal you can see that for a moment as the piston rod is pushed there will be a free path fro fluid to flow from the reservoir through the piston center into the compression area and at the same time any fluid could flow from the caliper back up through the master cylinder to the reservoir releasing any pressure.

[andy78rg]

When talking about the O-seal, is that a Scott Master Cylinder or a Clevland or a McCulley?

[minton]

How old are your brake calipers and when was the last time they were apart?

I'm going with this one.

In the past when I've torn down brakes for rebuild/inspection I have found many of the caliper pistons (Cleveland) to be partially or fully bound up in the housing either by corrosion, dirt, etc. so they never fully retract (when the peddles are released) to the stand-bye postion as intendended by the MFG. Normal servcing of the reservoirs by the back pressure bleeding method to the fill port cannot make the discribed event happen. Another possible thing would be contamination in the brake master cylinder, if both are behaving the same look to senario #1, If senario #2 occures on only one brake I'd look at the master cylinder first as there is a return hole in the internals that sometimes gets dirt in it..

Maybe an entire rebuild of the brake system is in order Costly in time and treasure but then you know what you have going forward.

Just a PS. There are some good tricks out there to get a "Frozen piston" out of the housing. One being a grease gun and "C" clamp.

Good luck

just my humble opinion.

[GAHorn]

I've blown a frozen piston out using compressed air. (Wrap it in an old towel first to contain the projectile, if you try that method, tho'.)

The caliper does not actually "retract" in a disc brake system. The piston merely applies pressure to the brake-pad...or it relaxes with no pressure applied. (However it should be free to do so with mere hand pressure.) The only reason a piston may "retract" is because of movement of the disc which might push the piston slightly back into it's cylinder/housing. The cylinder/piston should be completely disassembled, cleaned, inspected, and re-sealed each brake job. The majority of owners only replace the linings and re-install, however.