Hydroboost Question

[88F350idi]

Hey everybody,
I got my hands on a hydroboost setup from a 1994 F450 superduty and put in in my 1988 F350. I am happy with the added performance of the braking system. However, the master cylinder from the 450 is of a larger bore than my F350. This, i believe, has caused my brake pedal to be very hard, and has very little travel. I attempted to bolt my stock master to the hydroboost, but the holes for mounting are wider on the new one and my old one doesn't fit. Is there a different master i should be using? I believe my original was a 1 1/8" bore while the 450 one is a 1 5/16" bore. Also the superduty had rear discs with a load-sensitive proportioning valve, and i am running the stock SRW drums. Any help would be greatly appreciated.

 

[gatorman21218]

I have hydroboost on my truck. It doesnt matter what drums you have. Are you still running the F-350 M/C? Your best bet is to run a F-Superduty MC. Some people have elongated the bolt holes to make fit but for a bolt in fit you need the f-superduty one. Also there is a hydroboost specific intermediate pushrod you need as well. Also did you get the pedal assembly out of the F-superduty? if not then you need to either get it or drill a new hole for the pin on the brake pedal. A vacuum brake pedal's pin is about an inch to low, and while it can still fit over the hydroboost pushrod, it will bind up. You want the pedal to push the rod in level. On an 88 truck with a 5-speed, the only brake pedal that will work (other than welding a new pin) is from an 87-91 F-Superduty with a 5 speed

 

[LCAM-01XA]

I beg to differ on the master cylinder recommendation - the F-Superduty one is way too large for the small wheel cylinders we have, his factory m/c is much better match as far as fluid displacement and pedal travel go. Also, by using the F-Superduty m/c he's losing some mechanical advantage which partially cancels the increased piston force coming from the hydroboost, by retaining the factory m/c he keeps his good mechanical advantage and gets that much more brake assist thanks to the hydro.

To run a bricknose F350 m/c on an F-Superduty hydro one must slot the m/c mounting holes by 1/16" or so away from the bore. Also, a spacer may be required to insert in the m/c piston bore for the pushrod, if the hydro pushrod is shorter than the vacuum one. I have both of these mods done to my factory F350 m/c, and it works absolutely beautiful with the GM hydro I run.

 

[88F350idi]

I have hydroboost on my truck. It doesnt matter what drums you have. Are you still running the F-350 M/C? Your best bet is to run a F-Superduty MC. Some people have elongated the bolt holes to make fit but for a bolt in fit you need the f-superduty one. Also there is a hydroboost specific intermediate pushrod you need as well. Also did you get the pedal assembly out of the F-superduty? if not then you need to either get it or drill a new hole for the pin on the brake pedal. A vacuum brake pedal's pin is about an inch to low, and while it can still fit over the hydroboost pushrod, it will bind up. You want the pedal to push the rod in level. On an 88 truck with a 5-speed, the only brake pedal that will work (other than welding a new pin) is from an 87-91 F-Superduty with a 5 speed

Thank you. i am running the superduty master cylinder and it seems to be the problem. Which part are you referring to as "intermediate pushrod"? is this the piece that connects to the pedal arm? Thanks again.

 

[88F350idi]

I beg to differ on the master cylinder recommendation - the F-Superduty one is way too large for the small wheel cylinders we have, his factory m/c is much better match as far as fluid displacement and pedal travel go. Also, by using the F-Superduty m/c he's losing some mechanical advantage which partially cancels the increased piston force coming from the hydroboost, by retaining the factory m/c he keeps his good mechanical advantage and gets that much more brake assist thanks to the hydro.

To run a bricknose F350 m/c on an F-Superduty hydro one must slot the m/c mounting holes by 1/16" or so away from the bore. Also, a spacer may be required to insert in the m/c piston bore for the pushrod, if the hydro pushrod is shorter than the vacuum one. I have both of these mods done to my factory F350 m/c, and it works absolutely beautiful with the GM hydro I run.

Thank you for this info. It is what i was looking for. I will go ahead and slot out my master cylinder to fit. Did you have to do the mods to your pedal arm? mine seems to line up just fine (but i will double check)

 

[towcat]

80-91 trucks need the 87-91 F450 pedal arm to work correctly.

 

[snicklas]

I know this is in a newer truck (My Superduty), but with the hydroboost brakes,it does not take much pedal movement for the brakes to work. This is on a factory system, 100% since the superduties have factory hydroboost, but I have very good braking power at 1-1.5" of pedal travel. Driving any vehicle that has vacuum brakes after driving the truck (this is even my cars, A Neon and a Sable Wagon) it feels like there is something wrong with the brakes, since the pedal has to move so far for the brakes to work, and the braking power is so much less. That is one thing that may surprise a driver when they switch to Hydroboost. It is a much firmer brake pedal, with a great deal of braking strength. It is not a firm as vacuum with a failed booster (no vacuum condition) but much firmer than a working vacuum system. I know that if I drive the truck for a few days, and then get home and jump in the car and take off, I have to convince myself there is nothing wrong with the brakes in the car..... If I could, I would put hydro in the cars also...... I know that with the 8000 lb Excursion, with 4 wheel disk brakes and factory hydroboost, that I can stop it quicker than my Neon or Sable. I have stopped the Ex towing my 4500lb trailer without the trailer brakes and it felt better the F-150 we had towing a 2000 lb trailer and no brakes on the trailer.

 

[icanfixall]

The stock vacuum brake system with the smaller mc has 1000 lbs of hydraulic pressure in the system... The hydroboost braking system is able yo make 2100 lbs of hydraulic pressure so.... Using the hydroboost you are doubleing your available brakeing pressure. Use the larger 1 5/16 master cylinder and the brake peddle designed for it. You will have about 1 inch of brake peddle travel.... why go back to the smaller mc. The change in th pin height is going to increase your foot power advantage... Not decrease it... think about a lever and fulcrum advantage or a block and tackle. You will not feel any differance in lost advantage using the proper equiptment... This has been talked about... Kicked around... And plenty more... When you change out al the above mentioned parts you have twice as good braking now compared to the vacuum brake system you left in thw weeds.....

 

[88F350idi]

The stock vacuum brake system with the smaller mc has 1000 lbs of hydraulic pressure in the system... The hydroboost braking system is able yo make 2100 lbs of hydraulic pressure so.... Using the hydroboost you are doubleing your available brakeing pressure. Use the larger 1 5/16 master cylinder and the brake peddle designed for it. You will have about 1 inch of brake peddle travel.... why go back to the smaller mc. The change in th pin height is going to increase your foot power advantage... Not decrease it... think about a lever and fulcrum advantage or a block and tackle. You will not feel any differance in lost advantage using the proper equiptment... This has been talked about... Kicked around... And plenty more... When you change out al the above mentioned parts you have twice as good braking now compared to the vacuum brake system you left in thw weeds.....

Thank you. i do understand all this and it makes good sense to me. With a little more work i should be all set. Thanks for all the input everyone has given.

 

[LCAM-01XA]

Gary, I stand corrected - while what I said about mechanical advantage through the m/c bore size is still true, I forgot that you guys make up for it by moving the pushrod pin closer to the pivot. So, I would change my recommendation to this - if you use F-Superduty pedal assembly then run the F-Superduty m/c too. However, I still think that if one is utilizing the factory pushrod pin on the pedal (like I did), they are better off also keep the factory m/c...

 

[icanfixall]

The issues with the factory brakes on most every 250 or 350 truck is the rear auto adjusting brake setup... they just don't work... then you end up with a low peddle and the fronts doing all the braking... Thats not a good thing... The fronts overheat when towing and you get the sinking peddle at a stop lite... The peddle sinking is due to the needed volume required to fill the rear wheel cylinders because they are out of adjustment. To test this see where your parking brake sets... If its near the floor or more than 3/4 the way to the floor your rears are way out of adjustment. another way to test is apply the parking brake about 1/4 to 1/2 of the travel and test drive it... See if the brake peddle is higher up and the stopping is better. With a larger master cylinder more volume is being pushed into the braking syatem. That extra volume will easily fill the rear cylinders but... continue to self adjust the rears or the fronts will heat up... Warp and burn out much sooner.... I learned this the expensive way....

 

[LCAM-01XA]

Oh yeah, them stupid things - I hate them to the point where I removed the small actuator cables on mine, and the latch-levers are bent in such a way that there is absolutely no possibility of the ratchet wheels loosening (or tightening, for that matter) on their own. Every time rear wheels get pulled for whatever reason the drums come off too and the edges get deburred, and shoes get readjusted. My parking brake locks good and solid on the very first click of the ratcheting mechanism, and I have to revv the heck out the engine to make the truck move...

 

[gatorman21218]

just some advice- change your brake hoses out. I left mine alone when I converted and they locked up my calipers on the first test run. Dont forget the one in the rear. Its cheaper to change the hoses then to replace the calipers/ burnt up pads AND have to replace the hoses anyway

 

[LCAM-01XA]

And while you're doing the hoses, inspect the lines as well, and replace as necessary (or entirely) - that exact hydro unit that now stops my IDI, when first installed in my old Chevy promptly blew both front lines on the first good brake apply, and then the rear line on the second brake-stand (both times it happened in the safety of my carport). Gary wasn't lying when he mentioned the thing doubles your line pressure, so make sure the lines can handle that during a panic stop.

 

[u2slow]

You don't need the F450 pedal... you can modify the one you have.

Drill and tap a hole 3/4" higher for a 9/16" NF bolt. That becomes your new stud. Secure the booster pushrod with a nylock nut.