Long pedal and grabby

[Sixt8Chrgr]

I have a 68 with power drum brakes all the way around the car.

I have what seems to be excessive pedal travel, and the brakes grab like crazy when I am going slow. I have adjusted the shoes out for the pedal travel with no change. I have no idea what to do about the grabby brakes at slow speed. I want to keep things original, but these brakes really suck. I have fluid in the reservoir. Any ideas on the long brake pedal travel?

Thanks,

Lawrence

[Belgium R/T -68]

If air in the system is your problem I'm the guy with the experience.

Per

[dpixel]

If the shoes are installed backwards they will be grabby.  Mine rear shoes were like that when I bought my car.  I believe the shorter shoe goes towards the front(in my rear brakes anyway).

[Musicman]

Long Pedal.... Sounds like you are missing the residual pressure valves.

[Sixt8Chrgr]

Long Pedal.... Sounds like you are missing the residual pressure valves.

Where do they go?

[b5blue]

Power brakes have an adjustable rod between the booster and master that needs to be set properly. Grabby brakes could be old or weak return springs and or not lubing the rub points on the backing plates. I've seen backing plates that actually have notches in those flats (where the shoe rubs) and in that case file them flat. 

[John_Kunkel]

I would first check the pushrod adjustment that b5blue mentioned, especially since there is excessive pedal travel, if it's adjusted too short the booster will hit the MC hard when the pedal is first depressed; the pushrod should just touch the pocket in the MC piston without actually moving the piston.

A faulty reaction valve in the booster can also cause grabby brakes.

[Musicman]

Where do they go?

If the master is original, then the valves are built into the front and rear brake outlet ports. If the master has been replaced with a rebuilt or aftermarket unit, you no longer have them. They would have to be purchased separately and installed in the brake tubing's, one for the front and one for the rear (drum brakes only).

Also check the push-rod length as B5 & John have suggested... Unbolt the master and measure precisely, the depth of the piston from the masters mounting surface. Now start the car and measure the rod length from the mounting face of the booster. Ideally, when assembled the tip of the rod should be as close to the piston as possible without actually contacting it. If it is to far away then you will have excess pedal travel. If it is to long, it will actually be engaging the piston and not allowing it to fully return.... each stroke of the pedal will preload the brakes more and more, and eventually they will begin to drag or lock up all together. Then you will have to wait for the excess pressure to bleed off before moving again, at which point the process will start all over again.

[Sixt8Chrgr]

The master and booster are original, but have been off the car.

It sounds like the rod is not adjusted properly. I will look into that.

Thanks,

Lawrence

[b5blue]

Mine was pulling, that was strut rod bushings (and 1 rod bent) and it had this trick of if you really put the brakes on hard and kept them on all the way to almost a stop the pedal stayed down till you stopped (!) I replaced the front wheel cylinders as they were seeping a tiny bit. That firmed them up reducing travel but I had to change out the M/C to really get things right. So now I've overhauled all 4 drums and M/C to new specks even though next year I'm changing to 11 3/4 disks. Stopping is more important than going!

[Sixt8Chrgr]

Today while at a stop light the pedal just sank to the floor. The brake system light came on. I pumped up the pedal and things were better but the brake system is still grabby and I think I have excessive pedal travel.
I think the first thing I have to do is confirm that all the air is out of the system and confirm there are no leaks. While doing this I can figure out of the pads are on backwards and then finally confirm the rod to the m/c is adjusted correct.
Does this sound reasonable?

[b5blue]

Yup! 

[Musicman]

Today while at a stop light the pedal just sank to the floor. The brake system light came on. I pumped up the pedal and things were better but the brake system is still grabby and I think I have excessive pedal travel.

It's beginning to sound like "trapped air" in the tubing's or the wheel cylinders. If the brake system light came on, that means that the hydraulic safety switch located in the distribution block, slid far enough to the front or rear to activate the warning lamp. The only way that can happen in a fluid system is by compressing air that has been trapped within the system, or by discharging brake fluid through a leak in the system. It is possible for wheel cylinders to ingest air over time if no residual pressure valves are present in the system.
Sounds like a good thorough inspection is in order, as well as a complete brake system fluid flush. Old fluid absorbs air as well, so start at the fluid reservoir and just keep flushing the brake lines with new fluid until the old crap is gone... and the air with it.

[Sixt8Chrgr]

Thank you.

[b5blue]

Be certain they are adjusted fairly snug also, long but firm peddle travel is a sign of this. In the past I had one wheel in the front that would fail to adjust proper and that was a sign to crank on it. Mushie peddle is air in the system.

[John_Kunkel]

If the problem was air in the system the problem wouldn't be intermittent, the pedal suddenly sinking to the floor is a master cylinder problem...the MC is bypassing internally. Time for a replacement.

[b5blue]

 

[Musicman]

If the problem was air in the system the problem wouldn't be intermittent, the pedal suddenly sinking to the floor is a master cylinder problem...the MC is bypassing internally. Time for a replacement.

Very true John, but on the other hand a bad master shouldn't be so intermittent either, and it shouldn't cause the warning light to actuate... Normally, fluid would have to be moving downstream of the distribution block for that to happen. Although it is possible for a really bad master to literally recirc and give the same indication. There may very well be more than one problem here, but you have to start somewhere... I'm simply suggesting that he start with a good system flush and a thorough visual inspection of all the components first. The master can be tested well enough with the car sitting still in the driveway.

[John_Kunkel]

I disagree, every MC that has bypassed on me started out as intermittent, I (wisely) replaced them before that totally failed.

The warning light illuminates because only one of the pistons in the MC is bypassing, as long as one piston still provides some pressure there will be an imbalance in the systems and that's what triggers the light.

[b5blue]

For 35 bucks put a new...not rebuilt M/C on to be safe. 

[Sixt8Chrgr]

For 35 bucks put a new...not rebuilt M/C on to be safe. 

I want to try and stay as original as possible. Where do you suggest that I buy one?

I know that I can get my original rebuilt at whitepost, but I dont have to spend that kind of money either.

Any suggestions?

[b5blue]

I got a new (not re-built Doorman) from rockauto on the cheap= 35.00=made in china, works but not "exactly correct" looking. It stiffened up my peddle that last little bit (my 4 wheel cylinders are all new also) I'm switching to 11 3/4 disk as soon as money permits so I had no issue with looks or longevity to any great degree and as my car was a 318/904/7 1/4 axle new and is now 440/727 /8 3/4 bastard exact correct could never apply anyway. If your staying with drums get a better M/C than I did or get yours re-done.

[Sixt8Chrgr]

NAPA has an original rebuilt for $30 with the core. I actually have not seen it in person just online. If in fact what they have in the store is what they show online I will probably go that route.

Thanks,

[b5blue]

Do not give them your core until your certain it's repaired. 

[Musicman]

Replacing the master can't hurt anything, and it may very well be the source of all your problems. Outside of a leak in the system, there are only a couple things that I know of that would cause a pedal to suddenly drop to the floor. One of them is a bad master, which is probably the case here as John suggests... another is trapped air in a wheel cylinder with bad seals, that will expel some of the air under pressure. Flushing the system will ensure that any trapped air gets removed from the system, and will be required if you change the master anyway. Don't forget to inspect the rest of the system and components while your there... and remember... the newly rebuilt master will not have the pressure valves built into it... none of them do. Also remember that you need to prime/bleed the master before use.