Master cylinder location question 8

[dwaldron]

I installed disc brakes on a small machine...lack of space forced me to locate the MC below the calipers. I'm having a hell of a time getting the air bled out of the system. I understand that a fix would be to move the MC above the calipers...but I can't do that...so would a fix be to locate the MC reservoir remotely and above the calipers?

Dan

 

[MikeHalloran]

That should work better.



Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

That won't do anything. Your real problem is not bleeding the brakes properly. As long as you are not running low on fluid and pumping air into the system, a low mount M/C is not a problem. Try the catch can (or bottle) method and use two people. If that won't do it you will need to power bleed the system by putting pressurized fluid through the M/C reservoir and out to the brakes. Because the M/C is low, you may need to bleed the brakes once in a while.

 

[swall]

Did you pre-bleed the master cylinder?

 

[gpara]

I have used a vacuum system for years on hard to bleed brake systems. You can use an inexpensive hand pump vac. to romove the air from the system. I have converted a clear glass jar with 2 ports on the top to see the flow of fluid and act as a reserviour.
gpara

 

[dwaldron]

I picked up one of these hand vacuum pumps and it worked, but I still had air in the system. So I forced fluid back through the caliper with a syringe, and that purged more air out…so I think the system is bled. Now what I have is the pedal action seems soft for about half of its approx. one inch stroke, and then it firms up...is this normal? So the pedal travels a long way because of the pedal-cylinder geometry, which I don't think I can easily change due to my installed space limits.

Dan

 

[evelrod]

It still sounds like you may have a bit of trapped air in the system and/or a mechanical linkage problem.

As I see it, you still are not bleeding the system properly as Fabrico has indicated in the post above. Nearly all hydraulic braking systems of the early automotive era were of such configuration whereby the MC was mounted below the floor boards and below the wheel cylinders.

There are too many works on 'how to' available and, most, if not all, will usually get the job done---in your case, I think I would go to the 'power bleed' or, if that's too inconvienient/expensive---bleed the system as you would a hydraulic clutch...slow pumps with a catch bottle where the tube from the wheel bleed is submerged in fluid so that no air can get back into the cylinder. Just basics. You may need a helper, makes the job go faster---but you can do it by yourself with practice.

One thing else, 'reverse bleeding', while effective on some of my race car applications, is not a good idea on some of the more modern ABS systems since contaminated/overheated fluid can be pumped into sensitive areas of the ABS...even to the point where wheel calipers are bled prior to a pad change rather than force the fluid back through the system!

Rod

 

[dwaldron]

Thanks Elrod,
This is not an ABS system, so no worry there. I have tried the power bleed...if you mean forcing fluid through the system from the reservoir...I used a reservoir cap with tire valve stem in it and about 5-10 psi off my air compressor...I pushed the equivalent of two reservoir volumes through...and still got air out of the system with the vacuum tool and the reverse fluid with the syringe...seems like I have a tough one on my hands. I will try the “basic way” and see what happens.
Dan

 

[evelrod]

As long as your power bleed did not allow the master to run out of fluid, my guess is that you need to rebuild the MC if it is not new already. I had a NEW Girling MC that just could not be bled...finally replaced it and the system bled on the first try. Go figure. 'Parts is parts', for sure but, not all parts are equal.

Rod

 

[carnage1]

one possibility if all the air is out would be improperly adjusted brakes. with drum brakes if they are not adjusted propperly you use up some of your pedal travel to get the brakes where they shoud have started. With disk brakes something similar can happen if the disk is somehow able to wobble. (always double-check axel nuts on FWD cars after removing and BEFORE the long road trip you fixed the car for.)The wobbling motion will force the calipers back away from the disks and will use some of the pedal travel to move them back into contact.

 

[GregLocock]

As carnage1 and evelrod almost said, try forcing the pistons back into the callipers before you start, that can pump out a lot of air.


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[dwaldron]

Thanks Greg and Carnage1,
I will make sure the caliper discs are engaging the rotors and try the power bleed again.
Dan

 

[cnova]

Make sure that the caliper is mounted in the correct orientation.

Stupidity is the basic building block of the universe. F.Z.

 

[evelrod]

I don't usually forget one of my biggest 'peeves'---using silicon based brake fluids!!! Use of such fluid can and often does lead to 'compressability' issues and a 'long, spongy' pedal, symptomatic of air trapped in the system. In race car applications total brake failure was common, leading to obvious disaster. Sorry, I should have thought of that first off. I just have not been involved with silicon fluid in many years and it did not 'jump out' as a possible cause.

Rod

 

[dwaldron]

Rod,
I'm using good old DOT 3, so silicon is not an issue. But thanks anyway because I had considered using the silicon stuff and couldn't find anyone that liked it.
Dan

 

[evelrod]

Silicon fluid is often used in vintage and antique autos that 'sit around' a lot. As long as one is aware of the short comings (NOT advertised), the fluids primarily, IMO, 'pro' is that it does not absorb water...great for an antique, low use car. I rather dislike the stuff...it's dangerous in high performance use, in my opinion.

Rod

 

[dwaldron]

Rod,
Sounds like the silicon fluid works in standard master cylinders and doesn't require any special rework of components...is that true?
Dan

 

[evelrod]

I'm not sure. My involvement with silicon based fluid has been to 'remove' it. I have never used it in any of my antiques...I tend to stick with Castrol LMA in everything (Castrol Amber, in the old days)...well, almost...it's pretty hard to use any hydraulic fluid in my Model A ;-)

I guess that from the fact that when I did change out the Lockheed or Girling systems that they appeared to all be "standard" components...I renewed all the rubber anyway.

Rod

 

[swall]

I was the materials engineering supervisor at a major brake system supplier and yes, silicone DOT 5 fluid is compatible with all modern conventional master cylinders and disc calipers. We only had one customer that used silicone, however--the U.S. army. Two big drawbacks with silicone are its low water tolerance and its greater (than glycol) propensity to absorb air. The first results in any water finding its way to the lowest point in the system. The second can result in spongy pedal if the system is driven to a higher elevation and some dissolved air comes out of solution.

 

[dwaldron]

Swall,
It sounds like DOT 5 doesn’t have many supporters out there. I've always wondered why hydraulic fluid wasn't the usual fluid for brakes...but I guess it would require different rubber seals…am I right?
Dan