Brake bleeding hell

[Grand_Wag_85]

Working on getting my old IHC truck back on the road after an LS swap and extensive overhaul. The entire brake system is new and it’s been converted to Hydroboost with the C3 corvette MC.

MC was properly bench bled (block off style) and mounted on the truck with the block off ports still in place. When I started the truck the pedal would not move no matter what. Once I hooked the steel hard lines up the pedal went limp and not getting anything more than a drool at the bleeders.

Bought a pressure bleeder but the adapter is useless so I’m in the process of fanning one up. However I did use it as a reverse bleeder and got some action on the brakes. Unfortunately after about the 2nd bleed it goes back to being a drool.

This is the 2nd MC I’ve tried. Myself and several others are stumped. Any ideas?

[Yeller]

What brakes are you running? Sounds like the c3 has too small of a bore, not the first time I’ve run into this. My bronco does the same thing, had to go to a 1-5/16 bore cylinder to make the brakes work with vacuum booster on my bronco. With hydro boost you can run a much larger master cylinder successfully. Many of them use a 1-1/2” bore factory. On my J Truck with hydroboost I run a 1-3/8 bore from a mid 2000’s Chevy 1500. The corvette is typically 1” bore.

For reference my bronco has 3.5” bore single piston calipers and the rear are 3” bore single piston calipers. My J truck has the typical J10 front discs and 12” drums.

[Stuka]

Did you bleed the hydraulic side of the booster? It likely wont produce any boost until you do.

But you should be able to bleed the brakes with the engine off. Though if the hydraulic side is partially bleed, it may be tough to push the pedal as it can get pressurized air on the other side of the pedal.

Are you using the master that goes with that model hydroboost? And did you check the length of the rod from the pedal to verify its long enough to actually depress the plunger in the booster?

[Grand_Wag_85]

I am using the master cylinder that is recommended for that Hydro boost by the manufacturer. I believe he’s the same guy that supplies BJ’s Off Road. The bore on this one is 1 1/4 inches.

Disc brakes in the front and drum in the rear. Dana 44 front Dana 60 rear. Front caliper is 3 1/8 inches?

I did not hydraulically bleed the Hydro boost itself. Didn’t know that I needed to.

[Yeller]

You shouldn’t need to hydraulically bleed the hydroboost to bleed the brakes. Really need to verify the bore size, I’ve never seen a 1-1/4 bore corvette master cylinder most of them are 7/8 or 1” bore, I’ve seen 1-1/8”, but they are aftermarket. I have looked high and low for a Corvette style master with a 1-1/4” bore and never found one. Regardless a 1” bore should supply the brakes you describe.

Stuka did bring up a good point that I forgot to mention. There are 2 styles of master cylinders in that have different pistons. There’s a flush type for most hydroboost and a recessed type for most vacuum boosters, but you said with the plugs in the master the pedal was rock hard, so that is not it unless the piston is depressed by the booster.

If the push rod is too long and pressing the piston in on the master at all, it will block the fluid from flowing to the wheel cylinders or even back to the master when you release the pedal. I’d verify that your pedal is not pushing on the booster at all by disconnecting the push rod from the pedal, that will cause the same scenario. You can also loosen the master from the booster and see if that helps. I have had to make spacers to go between the booster and the master to get the interface between the 2 correct. One of them I did had to have a 3/16” spacer to function correctly.

Due to all of the issues with mix and match parts are why I always try to use a booster and master from the same factory application, start mixing and matching it gets weird fast.

[Grand_Wag_85]

Disconnecting the rod from the brake pedal yields no results.

We hooked up a plastic bleeder kit just to see if there was flow and there is flow when the pedal is pressed, and the flow returns when the pedal is let go.

When I was bench bleeding the master cylinder, it would shoot fluid vertically, as it should, if the lid was not in place. Now that it’s hooked into the truck it does not do that anymore.

[Mopar_guy]

Stuka is right. You need to bleed the hydroboost as well. I went thru the same thing and after all the messing around with 4 different masters, once I got the engine started and bled the booster, it bled out fine and worked great. For some reason, you never hear about having to have the booster filled with oil to bleed the brakes out properly.

[SJTD]

Idunno.

Seems that if you can't bleed the brakes with an unbled hydroboost then if you were to blow a hose and run out of hyd fluid your brakes wouldn't work???

I don't buy it.

[sierrablue]

Idunno.

Seems that if you can't bleed the brakes with an unbled hydroboost then if you were to blow a hose and run out of hyd fluid your brakes wouldn't work???

I don't buy it.

I agree.

Dad's '96 Mustang has hydroboost from the factory. Just like with the vacuum brakes, it's hard to push when it's not running, but it's still got brakes. I mean I've never driven it without any fluid in it, but it uses the same pump as the power steering. My dad said one time autocrossing he was a little low on fluid, and all that really changed was that when he raced up to a cone and hit the brakes, the steering got hard. Topped it off and it was fine.

Brakes really don't feel that different when it's not running; it just gets rock hard way sooner.

The hydro is just the booster. There's a piston that goes into the piston in the master, that when the engine is running supplies a boost to give your leg on the pedal more mechanical advantage.

You couldn't legally run it if you had to have fluid in the booster to have brakes. Nor would it be safe.

[Grand_Wag_85]

Finally got the brakes bled out. Bought one of those pressure bleeders, and ended up having to fabricate an adapter for the master cylinder.

I am having an issue now where the brakes are dragging while driving. It is the front brakes. It is as if the Hydro boost is stuck on ever so slightly.

[Yeller]

It could be. I fought that with mine until I just put the master off the van it came from on it.

[ntsqd]

I don't really know how I got along without one of these, but now that I have it I check every m/c install.

https://www.amazon.com/dp/B0BDDBNDBX