I have an 87 4 wheel disc. I've replaced the motor with a carb. My brakes are firm when not running but pedal hits the floor while running. I've replaced the calipers, hoses, master, and booster. I've bled them about 20 times and still no brakes. I'm assuming it's a vacuum problem but have no obvious leaks. I'm at a loss, any help would be appreciated.

 

Did you bench bleed the master? If not, it might have air trapped in it. Could also have calipers upside-down.

 

Oh forgot to include that, yes bench bled too.
I'm sure calipers are on correctcorrectly. Engine has a bigger cam, I don't have specs, but was thinking not enough vac because of cam

 

Sounds like...
The Curse of the Delco-Moraine Brake-Calipers!


Just try some quick searches here and via Google for:
"Delco-Moraine rear Brakes".

Good luck!

 

Vacuum assist is about 2/3 the total force that goes into moving the brake pedal, with your foot doing the other 1/3 of the force. If the pedal still moves easy with engine running then you have plenty of vacuum assist. I don't think that's your problem.

Vacuum assist goes away when engine is off and your foot does 100% of the work. That's why the pedal gets hard after engine shut down -- you're feeling the real force needed to move the pedal. That's normal.

Brakes don't compress fluid, it just pushes fluid out of the master cylinder into the brakes. Then when brakes are released the fluid moves out of brakes back into master cylinder. If pedal is going to the floor then either,

1. Air is still in the system WHICH DOES COMPRESS, so your pedal movement can be going into squishing air and not moving the brake fluid and you run out of pedal travel. This is most common reason why pedal goes to floor.

2. Master cylinder isn't moving the fluid. Basically it leaks. Or maybe it is moving fluid but there's a leak downstream and you just don't know it yet. You'd probably see that unless it was a piston seal leak into the brake booster, which can happen, but the fluid level would drop in the master cylinder too.

3. The pedal mechanical linkage, bushings, pivot have a lot of slop and most the pedal movement is going into taking up slop and not actually moving the piston in the master cylinder. Hey, it's an old car, stuff wears out. The pedal ratio is 3.3:1 (foot move 3.3 times more distance than the pushrod that's actually pushing on master cylinder). Just to put it in perspective, the total piston throw is something like just 1 inch, so you can imagine how just 1/4 inch slop in the linkages can make the first 25% of pedal movement totally worthless for actual braking! That would put you near the bottom of pedal throw during normal circumstances.

4. Something is expanding when pressure is applied to the brake lines. The master cylinder only moves so much fluid. If the hoses are old and expand then there will be less fluid available to move the brakes. There are 5 hoses in the system: One at each front brake, One at each rear brake, One between axle and body.

5. Brake pads are too far away from brake rotor. The brake pads normally sit close to the rotor, and the master cylinder moves enough fluid to make the brake pads clamp to the rotor. If the pads have too much gap to rotor, then the master cylinder (and foot pedal) will run out of travel before the pads clamp hard. There's an adjustment procedure to set the brake pads at the proper distance from rotor.

 

Check the vacuum output from the motor. I had the same problem on my last motor and it all came down to the motor being too weak to pull enough vacuum.

 

Vacuum assist has nothing to do with pedal over travel.

 

 

Lol!

That reminds me of how much I was a Prankster.

When I was Young and full of energy...
I made a small Jumper-Harness that would fit most 1980s and 1990s GM Vehicles.
Actually, I must have made at least 40 or 50 of them.

These Jumper-Harnesses allowed me to connect the Brake Pedal Switch to the Horn Circuit under the Steering Column.
My friends hated me for a while!


When they got tired of that, I modified the the Harness to include a miniature Receiver from a Remote Door Lock Receiver.
Now I could turn On or Off the Jumper-Harness with a Remote Door Lock Transmitter.
I would intermittently activate it...
This drove my Friends absolutely NUTS!
A few days would go by... Just when they though that all the Horn honking was over...
I would reactivate it again!

 

Start isolating parts of the system. You proved your power brake booster is working correctly already. Start with the MC. Buy a couple of plugs that will plug off the MC outputs. With the plugs in place pedal should be rock hard even with the engine running, if it is the MC is fine and it's bled. If not then either you have air in the MC or the MC is NG. Usually if the MC is NG the pedal will be hard but then sink with the plugs in. If the MC checks OK install the frt. brake line in the MC and leave the plug in for the rear and see what the pedal is like. Same drill if it's OK proceed to adding the rear line and see what happens.

 

This is SO TRUE that it positively HURTS to see people outsmart themselves over it.

If the pedal ever goes to the floor, EVER, you have a BRAKE problem, NOT an assist problem. PERIOD. Doesn't matter in the least WHEN it does or doesn't go to the floor; if the pedal ever goes to the floor, EVER, you have a BRAKE problem, NOT an assist problem. PERIOD.

If there's anything you don't understand about the above statement, any letter in any word that needs further clarification, do not hesitate to ask for clarification. Do not argue, do not blather about "well I heard in the McDonalds parking lot last Friday night", do not tell us "I read on the Internet and you can't post anything on the Internet if it's not true", NOT NOT NOT NOT NOT NOT NOT NOT. Read, understand, and follow the above precept. Learn it, live it, love it.

You can largely divide the brake system in half: the "brakes" proper, and the "assist". The assist, whether it be vacuum like our cars came with, or hydroboost, or electric, or WHATVER, can be responsible for any number of assist-related issues. Mostly, these would boil down to, NO assist, NOT ENOUGH assist, uneven or unpredictable assist, or TOO MUCH assist (Frods used to occasionally have this happen: you'd press the brake pedal and it would go to the floor BY ITSELF and lock up the BRAKES PROPER if they worked), and such as that.

But:

ANY assist problem CANNOT POSSIBLY be the cause of the pedal going to the floor without stopping the car.

Given this FACT, there is a whole long list of problems that assist problem CANNOT POSSIBLY cause:

• Low pedal
• Go to the floor pedal
• Pulsations when coming to a stop
• Squealing when the brakes are pressed or released
• Brake fluid appearing outside of the brake system
• Brake fluid disappearing (although a combination of a faulty master cyl and faulty booster can produce this... but NOT a faulty booster by itself)
• Brake fade (where you're driving at highway speed and have to come to a sudden stop, immediately return to highway speed and have to suddenly stop again, etc., and after 2 or 3 or 4 iterations of this, you can press on the brakes as hard as you want and they don't really seem to do as much stopping as they did the 1st time)

In case Qwk or I haven't been sufficiently clear, let me repeat:

So let's make ANOTHER list, except this time, let's list the things that CAN cause what you're experiencing. I'll list them in the order that you should attack them in. IOW, from MOST likely, to LEAST likely.

• Rear brakes not working (improperly adjusted, rotors too thin, etc.)
• Master cyl has air in it from improper / inadequate / non-existent / etc. bench bleeding
• Rubber lines fornicated (expand when pressure is applied and don't act like hard steel the way they're supposed to)

I think that's about it. Either your rear brakes, which that system (Delco-Moraine cast-iron calipers) is well and widely known to be a total and irrevocable POS, and subject to this, are holding the pads SO FAR AWAY from the rotors that it takes more fluid than your MC can send their way to generate actual pad —>rotor contact; or; you have air in the MC.

Go fix those 2 things. Don't come back here about "it's new", "mechanic said", "it's too hard", "I thought", "I can't afford it", "is the only answer to any problem to just buy new parts", or ANY of that lame-a$$ mealy-mouth lazy LOSER crap. Go FIX THOSE 2 THINGS, making sure that they're REALLY FIXED and not just that you pretended to lamely distantly reluctantly halfassedly go through the motions for about a tenth as long as it would take to ACTUALLY FIX the problem you ACTUALLY HAVE, and tell us what happened.