| Dustbowl | -- 02-17-2020 @ 12:13 PM |
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I had to replace my 39 Ford master cylinder because of crack and leak. I bench purged the new one before installing. Then I used a power bleeder to purge about a quart of new fluid thru each wheel. But the brake pedal is still soft after this. I have done this bleeding successfully before and went exactly by instructions this time keeping about 25-30 psi on the power bleeder. I noted this time that there was a seemingly low flow of fluid out of each bleeder (done one at a time). It took about ten minutes of flow to collect one quart of fluid in my receiving jar. I have three specific questions for you: 1. Is the flow rate I observed normal or low in your experience? 2 If low, what could cause this? 3. If low, could this slow rate fail to purge air out of high spots in the hydraulic system? thanks Phillip
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| kubes40 | -- 02-18-2020 @ 4:57 AM |
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I too use a power bleeder on all of my projects. I start with 20#. The flow is smooth - difficult to describe in words. However, all of the air is forced through. I place a translucent line over the bleeder vale with the "open' end in a jar of fluid. That way, it is easy for me to see when the bubbles stop. If you still have a soft pedal after installing a new master, I'd suggest the master is defective. That is of course assuming your system was good prior to the (first) master becoming problematic. Mike "Kube" Kubarth
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| 37RAGTOPMAN | -- 02-18-2020 @ 6:10 AM |
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HI try just applying a little pressure on the brake pedal and hold it there, and see if it goes down if it does the master cylinder is defective, [brake fluid is passing the piston,bad seals ] also make sure you have some free play in the brake pedal rod, so piston returns,fully 1 inch of free play is normal also test your power bleeder, to see if brake fluid is not restricted, dirt etc, I do it the old fashion method by foot, with a clear hose and bottle, and with a helper, hope this helps 1937 RAGTOPMAN
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| TomO | -- 02-18-2020 @ 6:57 AM |
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Are you using DOT 5 fluid? A pressure bleeder is not recommended for DOT 5 fluid as it can introduce air into the fluid. If you are using DOT 3, 4 or one of the newer synthetic fluids, are you still getting air out of all of the wheel cylinders? if you are, the master cylinder is defective. You should be able to purge the air from the system using about 1 pint of fluid. If you are not getting air out of the system and still have a spongy pedal, the master cylinder or one of the wheel cylinders could be bad. Tom This message was edited by TomO on 2-18-20 @ 9:00 AM
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| Dustbowl | -- 02-18-2020 @ 8:55 AM |
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Thanks for useful replies but I'm still hopeful to hear from someone who has info on flow rate with power bleeder.
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| TomO | -- 02-19-2020 @ 9:53 AM |
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It is almost impossible to define the rate of flow when using a power bleeder. There are too many variables to give you a good answer. If you believe that the flow is less than when you have someone pump the pedal for you, then you could have a problem with the master cylinder. Tom
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| Dustbowl | -- 02-23-2020 @ 12:41 PM |
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My brake problem remains a mystery to me even after I am sure there is no air in brake system. My overall situation is as follows after installing new master cylinder: -brake shoes adjusted well -have bled brakes twice - about a pint each wheel both times - with power bleeder -there is no leak in system as 15 psi holds steady in the power bleeder -have adjusted the plunger from minimum to maximum with no significant change in results after all this, my brake pedal goes all the way to the floor using only hand pressure!!!!!! Separately, I adjusted all brake shoes tightly against the drums. In this case, the pedal goes about half way to the floor and stops hard. It does not seem "spongy" before stopping and no "fade" when holding pressure on the pedal. So my guess is that master cylinder is not pumping any/much fluid during part of its travel but I'm not confident in this conclusion. My next step is probably to replace the master cylinder again but first I would like to know is any of you come to a different conclusion when you consider the above information. Many thanks
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| 37RAGTOPMAN | -- 02-23-2020 @ 1:08 PM |
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HI how old are the 3 rubber hoses, they might be expanding giving you a soft pedal , it does not take much, have somebody pump the brakes and check the rubber lines,also try sliding the rubber boot on the master cylinder back 'do you see any brake fluid , if so replace the master, brake fluid is by passing the piston, like I tried to explain my last post. also I replaced the wheel cylinders on a 1938 Woodie I was doing a brake job for a customer, I had replaced all the wheel cylinders, bleed brakes , could not get a pedal went home,. next day removed the front wheels to find out the NEW REBUILT WHEEL CYLINDERS, were leaking, had filled up the grease retainer cups, WITH BRAKE FLUID, ARE YOU LOOSING BRAKE FLUID, called the supplier and return them, and bought 2 BRAND NEW wheel cylinder, PROBLEM SOLVED, hope this helps 1937RAGTOPMAN you have to be care full were you buy your parts, This message was edited by 37RAGTOPMAN on 2-23-20 @ 1:13 PM
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| kubes40 | -- 02-23-2020 @ 2:01 PM |
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Dustbowl, It would be nice to know what you had BEFORE you replaced the master cylinder. It's possible you have a defective cylinder. It happens. I have experienced a number of dual reservoir cylinders wherein the fluid goes between the reservoirs but not too much out to the wheel cylinders. While the rubber hoses can cause this symptom, it's highly unlikely. I'd suggest (without knowing why you had replaced the cylinder) that you go to your local NAPA and buy a NEW master (about $52) and swap out the one you currently have. Mike "Kube" Kubarth
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| Dustbowl | -- 02-23-2020 @ 3:20 PM |
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I forgot to mention that the brakes were working perfectly before I had to replace leaking master cylinder. So I believe it not an issue of expanding rubber hose or similar faults which would have shown up before i replaced the master cylinder. Does this automatically mean my new master cylinder is bad or did I do something wrong when installing it?
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| kubes40 | -- 02-23-2020 @ 4:18 PM |
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That's what I needed to know! Go get a NEW master cylinder my V8 friend. By the way, there's really no need to bench bleed a single reservoir master - especially when power bleeding. When you have the pressure on, pump the pedal once or twice before you bleed off the wheel cylinders. Mike "Kube" Kubarth
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| Dustbowl | -- 02-23-2020 @ 5:59 PM |
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Ok, I took out the master cylinder and tested on the bench. It pumps an amount of brake fluid equal to the piston displacement with a stroke equal to brake pedal movement. It creates very high pressure of outlet is blocked, and the is no "fade". Or piston slowly slipping towards end. And there s no leak So I conclude the M C was working well in the car That leaves only three reasons I can think of which would result in the brake pedal going to the floor without expanding brake shoes enough! - Brakes not adjusted properly, which I don't think is the case - Air in system, which I also doubt, or - Expanding rubber bosses Where else can pumped fluid be going? Any ideas? Puzzled dustbowl, 1934 Okie
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| kubes40 | -- 02-24-2020 @ 5:27 AM |
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If I understand you correctly, the brakes were working fine prior to swapping the master cylinder (sans the leak). So, if I am understanding correctly, the only thing that has changed is the master. Yes? Get a different master.... Mike "Kube" Kubarth
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| TomO | -- 02-24-2020 @ 7:01 AM |
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I agree with Mike. You cannot apply the same pressure to the MC piston when it is out of the car as it gets when you use the leverage of the brake pedal. A few years ago we went through 4 Chinese MC before giving up and asking the parts house if there were any made in the US. He ordered a re-manufactured MC and that worked fine. Make sure that you adjust the pedal to give 1/4" to 1/2" free play measured at the pedal. Any less will cause the brakes to lock up and more will cause a low pedal. There is no need to bench bleed a MC, but I do it to make sure that the new cylinder is functional and to speed up bleeding. I do not use a pressure bleeder. Tom
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| deluxe40 | -- 02-24-2020 @ 8:22 AM |
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On the chance that you had two simultaneous problems, a friend just spent over a year changing brake parts (cylinders, hoses, lines, shoes, fittings) only to discover that the soft pedal was caused by his brand new Chinese brake shoes. The metal shoes to which the linings are attached are all 3/16" too short where they enter the wheel cylinders. Tough to find and not easy to measure, but a set of genuine Ford shoes fixed the problem. This message was edited by deluxe40 on 2-24-20 @ 8:26 AM
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