vacuum assist brakes, not much brake pedal pressure after starting

[toadster]

after getting the coyote fired up, I have a vacuum hose connected to the whitby power brake booster, which from the photo below is vacuum #2 port


183625 183624

no leaks in the hoses, no bubbles in the brake fluid - but I would think the pedal would get harder to press or give more feedback when the engine is running

using the Whitby Brake Kit https://www.whitbymotorcars.com/product/power-brake-kit/

before the engine start - the pedal was firm
any ideas why the pedal pressure doesn't have much resistance now? e.g. pretty much goes to the floor...

[michael everson]

You might still have air in the system. It does travel much further with a booster though. You will find when you drive you will never push the pedal as far as you do when its sitting.
Mike

[toadster]

You might still have air in the system. It does travel much further with a booster though. You will find when you drive you will never push the pedal as far as you do when its sitting.
Mike

I'll have to double check again, thanks Michael :)

[CraigS]

The pedal will always push down more easily and further once engine is running and providing vacuum. If you have a daily driver w/ vac assist brakes you can do a test. Hop in the shut off car and push the brakes 3-4-5 times until the pedal comes up and is harder to push. Now start the engine...push the brake...feel the difference. We don't notice this very often now because the system seal better than they used to so each time you hop in the car there is still some vacuum there so pedal feel doesn't change. And there are also many other types of assist systems in use. Ages ago we used to see Mike's comment w/ a certain model Lexus. It had a lot of assist and so a very light effort needed to use the brakes. We were all in our 20s and 30s and felt like we could never get the air out when bleeding the system. Turned out that we were probably pushing 2-3 times as hard while bleeding as we needed to when driving.

[toadster]

The pedal will always push down more easily and further once engine is running and providing vacuum. If you have a daily driver w/ vac assist brakes you can do a test. Hop in the shut off car and push the brakes 3-4-5 times until the pedal comes up and is harder to push. Now start the engine...push the brake...feel the difference. We don't notice this very often now because the system seal better than they used to so each time you hop in the car there is still some vacuum there so pedal feel doesn't change. And there are also many other types of assist systems in use. Ages ago we used to see Mike's comment w/ a certain model Lexus. It had a lot of assist and so a very light effort needed to use the brakes. We were all in our 20s and 30s and felt like we could never get the air out when bleeding the system. Turned out that we were probably pushing 2-3 times as hard while bleeding as we needed to when driving.

that's what I assumed as well, but ever since starting the engine with vacuum hooked up I cannot get the pressure up even when the car is off... tackling some engine start issues currently, but will do another brake bleed - maybe a first start induced an air bubble as I didn't bench bleed the master, but the whole brake install was some time ago...

[CraigS]

Not sure what type MC you have but be sure you are getting full travel so the piston(S) move until they bottom out. Another thing to check is pushrod clearance. W/ a vac booster you actually have 2 pushrods. One is pedal to booster. The other is booster to MC. There is usually a somewhat tedious procedure to go through to measure booster pushrod length. Usually that pushrod has a threaded cap on the end that screws in and out to change it's length and jam nut to lock it down. The clearance is generally in the .010" to .015" range. If that clearance is too large your pedal needs to move too far before the MC is actuated.

[toadster]

Not sure what type MC you have but be sure you are getting full travel so the piston(S) move until they bottom out. Another thing to check is pushrod clearance. W/ a vac booster you actually have 2 pushrods. One is pedal to booster. The other is booster to MC. There is usually a somewhat tedious procedure to go through to measure booster pushrod length. Usually that pushrod has a threaded cap on the end that screws in and out to change it's length and jam nut to lock it down. The clearance is generally in the .010" to .015" range. If that clearance is too large your pedal needs to move too far before the MC is actuated.

I believe the whitby kit come with a 95 Windstar MC, I did get a tool to measure the pushrod https://thefactoryfiveforum.com/showthread.php?34952-25th-MKIV-9772-Build-NOR*CAL-prepping-for-first-start&p=454130&viewfull=1#post454130

still fighting with spark on the coyote, hope to get the brakes bled after that

[toadster]

here's the measurement I used with the master/booster gauge - had a very slight gap between the booster pushrod and the gauge as per spec

190146

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still spongy feel on the brakes, about 4 inches of travel until the brakes bite... a bit stumped and would love to get this figured out before I seal up the footbox!