HammerWrench Posted November 29 Posted November 29 Sorry, I tried looking for the topic first. My 83 f250 7.5 has the booster and master cylinder pictured. The booster holds plenty of vacuum. The brake pedal acts like manual brakes. When I press very hard, the brake dash light comes on. Is this master correct? The LMC book says it may be a manual one which could explain why the brakes act how they do. I have bled them and there was no air in the system. The set up was in it when I bought it and are shiny, so replaced recently. Didn’t know if it was the wrong set up or maybe there was an unseen issue like air in the system. I saw there is another master with a plastic reservoir that could possibly be the correct one for power brakes. Any guidance?
Ifitaintbroke Posted November 29 Posted November 29 13 minutes ago, HammerWrench said: Sorry, I tried looking for the topic first. My 83 f250 7.5 has the booster and master cylinder pictured. The booster holds plenty of vacuum. The brake pedal acts like manual brakes. When I press very hard, the brake dash light comes on. Is this master correct? The LMC book says it may be a manual one which could explain why the brakes act how they do. I have bled them and there was no air in the system. The set up was in it when I bought it and are shiny, so replaced recently. Didn’t know if it was the wrong set up or maybe there was an unseen issue like air in the system. I saw there is another master with a plastic reservoir that could possibly be the correct one for power brakes. Any guidance? Looks identical to mine, which is factory power. The plastic one is for the later trucks. To my knowledge Bullnoses never had them.
viven44 Posted November 29 Posted November 29 56 minutes ago, HammerWrench said: brake dash light comes on Brake light trigged by proportioning valve. Usually is a sign there is imbalance between front and rear brakes in the proportioning valve. Probably needs to be bled again. 57 minutes ago, HammerWrench said: The brake pedal acts like manual brakes. I have this problem on my 78 Bronco... the brakes work fine but just really hard to press. Big Red, the 85 F250 I had was the same way. Could never figure out why. If you get to the bottom of this, I'd be interested to know what worked.
HammerWrench Posted November 29 Author Posted November 29 48 minutes ago, Ifitaintbroke said: Looks identical to mine, which is factory power. The plastic one is for the later trucks. To my knowledge Bullnoses never had them. If I pump it 3 times quickly, it will build some pressure then bleed back down to manual feeling. Maybe that’s air in the lines. I’ll try bleeding them again until the new fluid comes out. Not seeing any leaks under it and the pedal can hardly hold it when the choke is on.
ArdWrknTrk Posted November 29 Posted November 29 If you're under there bleeding brakes remember that the multifunction valve needs to be held centered while the bleeder at the wheel is open. One of its 'functions' is to shift over and stop fluid from being drained by the front or rear branch exclusively. You might also try adjusting the rear drums manually. You'd be surprised how much pedal can get used up just getting the shoes to make solid contact. Bradley is 100% correct. The aluminum masters with plastic reservoir started in 1987.
HammerWrench Posted November 29 Author Posted November 29 I have no idea what the multifunction valve is or where it’s located. Can you point me in the right direction?
ArdWrknTrk Posted November 29 Posted November 29 Vivek incorrectly calls it the proportioning valve. It's a brass block beneath the driver with separate front and rear circuits. Note that the light duty (150 & Bronco) and heavy duty (250/350) have different bore master cylinders and different methods to re-center the shuttle in the valve that closes the warning light circuit and stops fluid from reaching one or the other set of brakes. It also looks like your master cylinder has a residual pressure valve that is supposed to keep a bit of pressure biased to the rear. I know you're not the one that changed the master cylinder, but confirming it works while bench bleeding the unit before installation is advised.
viven44 Posted November 29 Posted November 29 (edited) 19 minutes ago, ArdWrknTrk said: Vivek incorrectly calls it the proportioning valve. Sorry I was using layman terminology, not necessarily what is correct Ford terminology Proportioning valve - Wikipedia As to the valve I was referring to, in Ford speak, I guess it is called the Brake Pressure Differential Valve (the one with a harness connected to it which trips the brake light under an imbalance condition) per the documentation at Brakes..... and thank for correcting me Jim, as there is an actual proportioning valve as well which changes the rear brake pressure based on load in bed... I have seen these on trucks with sterling axles... not Dana 70HD or Ford axles (not sure why). Edited November 29 by viven44
ArdWrknTrk Posted November 29 Posted November 29 I'm not trying to be critical, just trying to keep us all on the same page. Sterling is Ford... 🙂 They can definitely be found on Dana axles in duallys as well. More a ride height (load sensing) valve that decreases rear braking force as the truck pitches forward. Brake bias is determined by master/slave ratio, and you can understand why the larger dual piston 250/350 brakes need more displacement from a bigger master. I'm still trying to grasp "acts like manual brakes" The pedal is extremely high and hard?
HammerWrench Posted November 29 Author Posted November 29 The pedal is low. Engages about 2 inches from the floor. Can pump to get a little more height but falls back down pretty fast. There is no bed on it, if that matters.
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