Rock hard brake pedal

Topic: Rock hard brake pedal (Read 2367 times) previous topic - next topic

Rock hard brake pedal

August 26, 2009, 01:13:22 AM

So I've installed a TC rear with its disc brakes. I installed a '95 GT MC on the stock booster. The MC was bench bled and the brakes are bled. The brakes work great for the first half of travel before it hardens up. Putting more pressure on the pedal doesn't get the brakes to lockup point or much more braking power. The master cylinder is still getting 18" of vacuum.

Why the sudden braking problems? Ideas? Google fails me.

Rock hard brake pedal

Reply #1 – August 26, 2009, 01:44:19 AM

I thought you had to swap the booster as well if you change the master? But, since you changed the master, did you change the prop valve? Bypass/disable the old one?

Rock hard brake pedal

Reply #2 – August 26, 2009, 01:49:15 AM

The stock block's shuttle valve is defeated when the proportioning valve is defeated so I have a T for the front brakes and an adjustable inline to the rear. No stock distribution block.

Rock hard brake pedal

Reply #3 – August 26, 2009, 07:41:40 AM

Did you adjust the master cylinder pushrod?

Rock hard brake pedal

Reply #4 – August 26, 2009, 10:09:07 AM

The pushrod depth came in basically the same position as the stock one that was pulled out so it got put in as is without turning it. The rear brakes are getting about 25% (are the adjustable valves linear?) at the moment.

Also, shouldn't the pushrod either make it lockup too soon OR go to the floor if not adjusted correctly? I see neither behavior. I cannot get enough braking force to make the car shift its weight forward so it isn't drivable like this.

Edit:
Is there a name for the cylinder the pushrod slides into? Just curious.

Rock hard brake pedal

Reply #5 – August 26, 2009, 05:24:47 PM

i didnt like the 1 1/16" 95 V6/GT master on my car either. even with a sn95 booster, i still had a super stiff pedal with the larger MC.
switched to a 15/16" 95 cobra MC for my setup w/cobra brakes.

Rock hard brake pedal

Reply #6 – August 26, 2009, 08:20:20 PM

So we pulled the MC off and checked the depth - its the same as my old one. We adjusted the pushrod a little as prior to all this there was slop in the pedal also. With less travel before it engaged, it still feels the same. I cannot get it to feel better with either 50/50 front/rear or 100/0. The brake booster pulls the pedal down if I pump the brakes then start the car.

I've got 73mm pistons up front and the TC (43mm?) in the rear. All brakes are moving then I press the pedal. None are grabbing with the brakes off.

Rock hard brake pedal

Reply #7 – August 27, 2009, 12:49:17 AM

Sounds like a problem with the master cylinder to me. I am no expert, but I believe the pedal should be almost rock hard with no assist to it. it definitely should not pull down by itself. Have you bled the brakes good?

Rock hard brake pedal

Reply #8 – August 27, 2009, 01:30:07 AM

I meant brake booster...it's rock hard until I start the car. The pedal drops a bit once the car is started. The booster appears to work properly.

Rock hard brake pedal

Reply #9 – August 29, 2009, 12:28:32 AM

If some one helps you resolve this problem, please post.

I Posted with this same problem over a year ago. Followed the rear disc upgrade to a "T". If I literally stand on the brakes the rears will lock up first, no matter where I set the valve (just happens sooner or later). SSCCAARRYY

Don't have any more time for my 85 TC, so it's been parked for a year.

I gave up and figured I need larger rotors in the front??? Meaning SN95 stuff. Don't know when I'll ever get back to it.

Rock hard brake pedal

Reply #10 – August 29, 2009, 01:54:26 AM

So you still have Power brakes? And I assume it's a standard vacuum one? Sounds to me like you have a bad booster.

TED

Rock hard brake pedal

Reply #11 – August 29, 2009, 10:44:59 AM

I should update this thread.

It seems to magically have fixed itself overnight (Worrisome? A little.). After continuous tightening fittings under the hood, rebleeding many times, messing with the proportioning, driving it around, over night it showed improvement. I then dialed in the proportioning and can get the wheels to skid upon applying a lot of pressure on a road that grips pretty well. I can get the car to "nose down" without too much effort on the brake pedal. The pedal goes about 2/3 down before it locks - the stock 1" bore MC "may" have still have worked in this case.

The 3 port MC, could the bottom port be blocked off and be plumbed up to the side/front brake port? Just curious. It doesn't take much more effort to get to "nose down" point over the stock master cylinder so I may just leave it alone but I may just try it one day to know how well the stocker works with this setup.

I'm happy with the braking performance of the car now, including the parking/emergency brake that actually works well compared to the stock drums. I just need to figure out what is banging around in the rear area before I'll feel comfortable with the vehicle.

Rock hard brake pedal

Reply #12 – August 29, 2009, 10:50:05 AM

I will also add that I get whistling up front when bleeding the brakes - but only when I have 25-50% fluid going to the rear (the more, the louder/easier whistle). Anyone heard of this? It may be the master cylinder but I'm thinking it's more likely the proportioning valve. The valve is the SSBC rear-only one. Nothing is leaking over the last couple days. I do not hear it during normal driving, only when pumping the brakes during bleeding.

Rock hard brake pedal

Reply #13 – December 27, 2009, 09:44:26 PM

Sad to hear I'll just have to "wait". I upgraded the rears to disc and the master to the crown vic, plus the SSBC Adj. Prop valve, and not cannot get any of the brakes to lock up. I can definitely get it to "nose forward", and it slows down reasonably, but it will not lock up before the pedal hits the floor. Granted, I've only tried a few times at speeds less than 30.

Oh well, must be a bubble in the line that cannot be cleared by bleeding.

Rock hard brake pedal

Reply #14 – December 27, 2009, 09:55:50 PM

How are you bleeding? Obviously there's either air or the fluid displacement isn't enough to lock them up - I had no trouble locking all 4 tires at at below 30mph by slamming on the pedal quickly and firmly.

I really need to go over my system one more time though as the brakes have worked well enough and the fluid hasn't dropped, yet I feel I have a bit less grabbing power than before the swap and adjusting the proportioning hasn't helped that.

I've had most luck bleeding on my own by just running a tight clear hose from the bleeder valve to a small container with the hose submerged in fluid - a good 5 pumps and the jar's full and the MC needs topped off. Of course, the proportioning is set all the way rear (50/50) during this to prevent the air from backtracking. Waste of fluid but the air all came out last time I had done it. This last time I didn't redo the fronts though as it got dark and wet and haven't had a chance since.

The odd thing is that when I take off, my brakes are spongy-ish the first couple times I use them and they're great from there on out until the vehicle sits again. I figure it's trapped air somewhere (likely the front due to the noticeable brake feel difference) but it's just weird.

Where do you work on your car at? It's going to be a rainy rest of the week.