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hard brakes

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Snake Oiler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Snake Oiler Quote  Post ReplyReply Direct Link To This Post Topic: hard brakes
    Posted: Mar/31/2022 at 5:34pm
All.  1971 AMC Javelin SST with 4 drum brakes.
New Lines, new brake cylinders, new Master Cylinder, Rebuilt Proportioning Valve (or whatever valve you want to call it that has a valve in it and goes to idiot light in dash), all shoes are good, springs and components are good.  New T-fitting for rear lines.  Rebuilt Power booster (sent core to TX to factory who rebuilds these "rare" boosters - they have no cores for 71 javelin - they said its a "One-off", so I sent it, and they returned it looking like new.  Brakes have been bled to the gills.

New Edelbrock torquer intake.  No leaks, good vacuum running straight off manifold.

VERY experienced mechanic says that he wants to put a vacuum canister on car to hold air pressure and tie that to the brake booster.  He says rebuilt brake booster might be bad.  No hiss and otherwise acts like its fine.  says brakes are too hard to push. 

Any ideas?  Thanks, Snake
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304-dude View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Mar/31/2022 at 5:44pm
New master cylinders have the wrong clearance with piston cup to actuator rod. You can adjust the apply rod to the booster to correct. My reasoning, is a factory master, has multiple washer packs to seat the piston slightly deeper in the bore. I had posted a picture of all the components to the master at the piston cup end. I do believe there was 2 washers and a clip to seat and hold everything in the bore.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: Mar/31/2022 at 8:58pm
Another thing to look at is the distribution block on the firewall (yes, the brass thing with the wire coming out of the top).  While you may have 'bled the system to death'...if during all the work the distribution block experienced a large degree of pressure differential, for whatever reason, the pin may have dropped, which would create a hard-pedal situation with everything else in the system properly sealed up.  Normally, this would cause the brake light to illuminate on the dash, but as old as these switches, wires and connections are, that's not a sure thing.  SO first thing I'd do is disconnect the block from the lines, remove from the car, gently put it in a vice and see if the pin on the switch has dropped. 
73 Gremlin 401/5-spd.
77 Matador Wagon 360/727.
81 Jeep J10 LWB 360/4-spd
83 Concord DL 4-dr 258/auto

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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Mar/31/2022 at 10:55pm
Experienced mechanic would know how to diagnose this stuff. New master cylinder? Check piston depth of the one you took off, compare to new one. 

A vacuum canister might help with a cammed engine with low vacuum but if stock, vaccum isn't the issue. Of course the booster could be done wrong, but there's ways to test.
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Snake Oiler View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Snake Oiler Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2022 at 8:18am
**UPDATE** thanks guys,  The distribution block has been rebuilt, pin in valve is acting normal, mechanic said brake action is fine when idling in park, but put in drive or reverse, brake pedal is hard.  this leads me to think it is a vacuum issue.  I will give him old Master Cylinder to measure piston depth. thanks for all your comments.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote bikerfox Quote  Post ReplyReply Direct Link To This Post Posted: Apr/01/2022 at 10:47pm
Glad to hear that you figured out the problem.

I was going to suggest taking a look at the material on your brake shoes.
Based on my experience, unless you are road racing, it's probably better
to use shoes that have organic material on them, as it's softer and will
grab the drums better, esp. if you've had the drums turned.  If you are 
running ceramic or semi-metallic material, based on my experience, it
will be harder to stop the vehicle.
1969 Rebel SST (1970-1987)
1968 AMX (2005-2011)
1969 SC/Rambler (2011-2019)
1970 Javelin (2019 to ?)"Jane"
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Snake Oiler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Snake Oiler Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2022 at 8:52am
update...there is plenty of vacuum, so I gave the original master cylinder to the mechanic and he said that the piston looks slightly longer.  I do not remember how he explained it but said that the piston may be going "farther?" in the cylinder and the vacuum hole going past the point where it needs to be therefor not letting the booster work properly.  He is going to take it out and measure the new against the old today.  Thanks for all replies - Snake.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Apr/15/2022 at 4:06pm
Originally posted by Snake Oiler Snake Oiler wrote:

update...there is plenty of vacuum, so I gave the original master cylinder to the mechanic and he said that the piston looks slightly longer.  I do not remember how he explained it but said that the piston may be going "farther?" in the cylinder and the vacuum hole going past the point where it needs to be therefor not letting the booster work properly.  He is going to take it out and measure the new against the old today.  Thanks for all replies - Snake.


Like I eaxplained, without the extra depth added by the oem washers, the piston will have a taller depth in the cup. This the reason why many with boosters will require adjustment to the apply rod, when replacing with remanufactured replacements. I discovered that after noting many reporting issues with instant apply of the brakes and lockups to the fronts. I disassembled my master to see what could be the cause. Having the cup closer to the apply rod, makes the piston engage by tight fit with apply rod, when installing.

By adding a proper thickness of washer to seat the piston a wee deeper in the bore, will correct without adjusting apply rod. If you don't have the proper depth tool for adjusting the rod, I think the washer needs to be about 1/16" thick, and able to fit within the bore, and apply against piston, with c-clip holding everything in place. Mine had 2 washers between piston and c-clip.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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Snake Oiler View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Snake Oiler Quote  Post ReplyReply Direct Link To This Post Posted: May/14/2022 at 6:09am
Well I got the car back this week.  Brakes still hard as a rock and car is hard to stop. Mechanic is a bit stumped. He said Master cylinder piston is same length as original and there were no extra washers on the original.  He is wondering if newly rebuilt brake booster is not rebuilt right or leaking somehow.  We have plenty of vacuum.  Again, distribution valve is rebuilt and acting correctly.  Not sure where to go from here... 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: May/16/2022 at 12:19am
Are you sure the brakes were bled properly?  I'll assume yes, but you know what happens when you assume things.  Sequence should be the farthest from the MC to the closest.  That means driver side rear first (yes, follow the lines), passenger side rear second, passenger side front 3rd, and driver side front 4th.  Then reset the pressure differential switch.
Another thing to check is to make sure all the vacuum ports (carb and manifold) have something on them.
You say the brake pedal feels fine at idle but gets hard when put in gear.  Have you watched the vacuum when you do this?  Is there some sort of vacuum module on the transmission that could be leaking only when not in park?  And if there is a vacuum module, what is the condition of the vacuum line to it?
Bill O.
70 AMX
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