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Spongy Pedal after Break Modification
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evernon
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PostPosted: Sun Apr 11, 2021 8:07 pm    Post subject: Spongy Pedal after Break Modification Reply with quote

I just replaced the rear brakes in my baja an haven't had any luck getting a firm pedal. I have been bleeding from all corners by both the pedal pump and mighty vac methods and haven't seen bubbles in the last 4 tries. The old brakes were two piston and were not up to the task of forcing 32" to skid. The new calipers are 4 piston willwoods. So I am wondering if my master cylinder is compatible with the new brakes. I think it is just a stock MC for bug with disk brakes. One thing that is interesting is that if I apply the cutting brake, the handle feels good and the brakes respond well. If I hold lthe cutting brake and then push on the pedal, the pedal is firm. I can release the hand brake and as long as I hold my foot on the pedal, the brakes are locked and the pedal stays firm. If I then release the pedal and re apply, the pedal is soft again. Pumping the pedal does not make any difference. Has anybody else solved this type of issue? Thanks!
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YDBD
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PostPosted: Sun Apr 11, 2021 8:32 pm    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

Does the cutting brake have a bleed valve? Bleed from there if it does, all the bubbles may be trapped in the circuit as it sits the highest.
The cutting brake piston may have bad seals/O-rings that also can be causing this.
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evernon
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PostPosted: Mon Apr 12, 2021 6:26 am    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

Yes, the cutting brake has a valve and I have bled from there several times now. The valve is not on the high spot though at the brake is installed at an angle. I could detach it and see if that helps. My feelings are that since the cutting brake feels right and the rear breaks work well with it, I don't see how there could be air trapped in that circuit. Another thing I don't understand is why I can't get the same response from the front brakes. Aren't the front and rear separate circuits? If so, why don't they lock up like they used to? When pumping the pedal, more fluid comes out the rear than the front per pump.
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Dale M.
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PostPosted: Mon Apr 12, 2021 6:34 am    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

Where is bleeder valve on calipers, at top position or low position...

Dale
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stevebaz
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PostPosted: Mon Apr 12, 2021 8:25 am    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

you may need a residual pressure valve to keep the wheel cylinders somewhat extended so it doesnt take so much fluid volume when you press the peddle. You increased the volume of fluid needed but didn't increase the volume of the master cylinder.
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evernon
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PostPosted: Mon Apr 12, 2021 1:50 pm    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

The valve on the cutting brake is at the top vehicle front but like I said the unit is tilted up on the back so the bleader is lower by about an inch. The new calipers have bleeders on 4 corners and I have bled them unmounted so they are tilted to achieve a high side. Manual says only to bleed the two that are high on each side. What should the gap be between pad and rotor? I was thinking the same thing that the volume of fluid from the MC isn't adequate to fully engage the rear calipers. The hand brake is working great! I haven't fooled with a residual valve yet, would it go in front of the cutting brake then? If this is the problem, why aren't the fronts fully engaging? Is there some sort of internal balancing in a MC?
Thanks!
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74 Thing
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PostPosted: Mon Apr 12, 2021 3:29 pm    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

You still have air in the system.

Jack up the front or the rear and bleed again.

Some put anti seize or silicone grease on the bleeder valves threads to keep air from going back in.
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Dale M.
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PostPosted: Mon Apr 12, 2021 3:37 pm    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

evernon wrote:
The valve on the cutting brake is at the top vehicle front but like I said the unit is tilted up on the back so the bleader is lower by about an inch. The new calipers have bleeders on 4 corners and I have bled them unmounted so they are tilted to achieve a high side. Manual says only to bleed the two that are high on each side. What should the gap be between pad and rotor? I was thinking the same thing that the volume of fluid from the MC isn't adequate to fully engage the rear calipers. The hand brake is working great! I haven't fooled with a residual valve yet, would it go in front of the cutting brake then? If this is the problem, why aren't the fronts fully engaging? Is there some sort of internal balancing in a MC?
Thanks!


None pad always contact rotors....

Dale
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evernon
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PostPosted: Sat Apr 17, 2021 8:35 am    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

Have a 2lb residual valve coming, hopefully this will firm up the pedal.
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cbeck
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PostPosted: Sat Apr 17, 2021 10:22 am    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

The next thing to concentrate on should be the cutting brake with the bleeder that is too low. You need to somehow get the bleeder to the high point. Unbolt and move it, jack up front of the car...
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dirtkeeper
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PostPosted: Sun Apr 18, 2021 1:06 pm    Post subject: Re: Spongy Pedal after Break Modification Reply with quote

My steering brake didn’t come with a bleeder , and was up high on the center and I struggled to ever get all the air out. I finally ended up bleeding out the actual cylinder on the steering brake, loosen the cylinder bolt and pressing fluid out and reclosing the cylinder . Sounds like it might make things worse but for me it was a hallelujah moment , I swear I was fighting that bubble for years and finally the pedal was solid and the steering brake was solid. I think I have a 2lb or 10 lb residual pressure valve before the steering brake,I forget at the moment but I do remember that it didn’t solve my brake sponge problem.
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