AuthorTopic: DO I have to pump my brakes???!!!  (Read 2766 times)

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Offline bartsworld

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DO I have to pump my brakes???!!!
« on: May 18, 2016, 05:11:29 PM »
Putting final finishes on my rat rod.  I have a brake question.  I have a master cylinder from a mid 80's GMC.  The resi is set up for disc/drum combo.  That was my original plan.  but I decided to go 4 wheel disc brakes.    I bleed the brakes and thought well that was easy.  But I seem to have to pump the pedal a few times to get the pedal firm.  Could this be due to still having some air bubbles in the lines? Or because I'm not running a brake booster?  I have 3/16" line and am going through a Adjustable PRV.    I fixed all the leaks and the floor has been really dry since.  IF I have to live with soft brakes and get use to pumping them to stop I think I will have to go a different direction....   Any experience would be greatly appreciated.  OHH, and the Master cylinder is mounted above the calipers.  SO I shouldn't need the 2psi BPV,  So I've read.

Thanks ya'll.
Bart

Offline lowboy

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Re: DO I have to pump my brakes???!!!
« Reply #1 on: May 18, 2016, 06:49:24 PM »
Bart, it sounds to me like you still have air in the lines. We used Chevy master cylinders in the racecars for years. Although it's a Ford master with booster I have 4 wheel discs on a disc/drum master and they seem to work well. I used the factory BPV for the Ranger. It doesn't like to do burnouts but that maybe the low vacuum from the cam as it pushes the rolling front tires.
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miraclepieco

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Re: DO I have to pump my brakes???!!!
« Reply #2 on: May 19, 2016, 02:03:28 AM »
^^^ Ditto the above ^^^

I suspect there is still air in the system.
« Last Edit: May 19, 2016, 02:06:49 AM by miraclepieco »

Offline Blackwater

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Re: DO I have to pump my brakes???!!!
« Reply #3 on: May 19, 2016, 08:32:20 AM »
^^^ Ditto the above ^^^

I suspect there is still air in the system.

That gets my vote on first glance.  Another possibility, although less likely, is that the volume produced by the master cylinder for the rear brakes is too small. If that is the case, when you pump the brakes a couple of times, you add fluid and thus pressure to the rear brake system and they apply late. 

Raise the car ad have someone apply the brakes, pressing the pedal only one time. Check to see if the front brakes will lock up but the back ones won't. If this is the case, you need to replace the master cylinder with one that supplies more volume to the rear system.  We had this problem when we installed rear disc brakes on my brother's Cutlass. A different aster cylinder, one for a four wheel disc Corvette, solved the problem.
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Offline hotwheels

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Re: DO I have to pump my brakes???!!!
« Reply #4 on: May 19, 2016, 04:14:31 PM »
Are all of your caliper bleeders on top? If any of the caliper bleeders are basically on the bottom, you won't get all the air out. If they are all up, I would look at bleeding them all again, could possibly use an entire bottle of brake fluid bleeding all the brakes properly. Also, start on the right rear, move to the left rear, then the right front and lastly the drivers front.
The 2# bpv you are talking about is used to stop fluid from rushing back to your brake reservoir. I use them on my builds, but they are not truly needed
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Offline bartsworld

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Re: DO I have to pump my brakes???!!!
« Reply #5 on: May 23, 2016, 02:11:47 PM »
Black water.  I did that last night and that is exactly what is happening.   Front brakes lock up great.  Have to pump the rear a bunch to get them to work.  Thanks everyone for the replies.  I'm going to order a master cylinder from a older corvette.   That should do the trick.


Offline Blackwater

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Re: DO I have to pump my brakes???!!!
« Reply #6 on: May 23, 2016, 05:46:47 PM »
You may still need an adjustable proportioning valve to get the front to rear brake bias just right.  Glad I could help.
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Offline bartsworld

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Re: DO I have to pump my brakes???!!!
« Reply #7 on: May 23, 2016, 06:37:09 PM »
I actually do have an adjustable proportioning valve in line now.  With the correct master cylinder do you think I need the residual valve?  the 2 psi ones in the back?

Offline Blackwater

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Re: DO I have to pump my brakes???!!!
« Reply #8 on: May 24, 2016, 07:15:54 AM »
It's kinda' optional.  We're NOT running one on the '73 Mach1 and it works fine. Same on the Cutlass.  If your master cylinder is below the calipers it might be needed, but on several of my old drag cars with similar setups we didn't use 'em.
Never use a minor caliber bullet on a major caliber adversary!!

miraclepieco

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Re: DO I have to pump my brakes???!!!
« Reply #9 on: May 24, 2016, 03:28:39 PM »
RPV's (residual pressure valves) should be used whenever the MC is below the calipers/wheel cylinders. 10 lb for drum brakes, 2 lb for disc brakes. Otherwise you could get backflow, and you might suck air back into the system when bleeding. Some proportioning valves are actually combination valves which also contain RPV's. Some MC's have them internally too, but it doesn't hurt to run inline ones just in case.

BTW, being inherently lazy, I have learned to love the "speed bleeders." They work super efficiently and they are ridiculously cheap. Just open the bleeder valve and pump your brakes - no helper or vacuum tool required. They are the only way I could finally get the air out of the old '40 Ford brakes on my sedan.

http://www.jegs.com/i/Russell/799/639600/10002/-1?CAWELAID=1710529773&CAGPSPN=pla&CAAGID=15769068431&CATCI=pla-196008289031&gclid=Cj0KEQjw94-6BRDkk568hcyg3-YBEiQAnmuwknV6bcxcANbE4EXfmksPOKbg-HFw62r6frH1yKQF0NEaAtFZ8P8HAQ
« Last Edit: May 24, 2016, 03:38:33 PM by miraclepieco »

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