Brake issues never resolved

[John S]

I would like some advice I have asked this question before but I hope I can get an answer on my brake issue. First here is what I have done. Every brake line has been replaced even though the rubber flex lines all three of them were only two years old I replaced them again. Front calipers have been rebuilt rear wheel cylinders are new distribution block cleaned and works properly I tested it by simulating a line break And the brake light functions properly as stated in the factory manual. Master cylinder has been replaced, power brake booster has been replaced, Along with the check valve numerous times. I have 20 inches of vacuum at idle so I do not believe that is the issue. Also the new power booster operates properly after testing it. I also took great care to check double check and triple check that the pushrod from the booster to the master cylinder was the correct length so it was not partially pushing on the master cylinder. The vehicle has been pressure bled, and also manually bled. It was bled properly I did have the hold off valve at the master cylinder pushed in while bleeding and bled it exactly as the factory manual stated. Right now the brakes again feel like drum brakes with no power assist. When you hit the brakes hard it will not lock up it will gradually come to a stop and it feels like brake fade.After talking with a few people who have tried different braking systems a suggestion was made to me to try something from a company called stop tech. New rotors and brake pads a person who did this to his vehicle said it made a big difference. I do not know if my rotors are bad can they be work hardened after all these years I don’t know, any advice would be appreciated. But as I stated everything has been replaced. Thanks for any help you can give me. John S

[stangeba]

When did you take delivery of this 1970 Monte Carlo?

Were the brakes acting like this that first drive?

Has there been any improvement or is it the same or is it worse than it was the first day?

I would ask what has changed but you already told us, EVERYTHING!

I still have the original booster and master. I rebuilt the master Cylinder probably 20+ years ago and replaced some of the lines. I do use DOT 5 fluid

If I were to grade the performance of my brakes, it would be a C-. I remember when it was new, I was not thrilled with the brakes and that was with OEM asbestos pads and shoes, (now gone)

I remember reading years ago someone got a replacement / rebuilt master and it was NOT the correct one. It may have had the wrong size bore and pistons or it may have been the rod that you already mentioned.

If you are not the original owner, God only knows who did what when and why! (That would make a great story line, Guess what the previous owner did to my car!)

Does anyone have specs for master cylinder's? May be a cool thing to have / find / share!

Good luck, Bruce

[stangeba]

Did a GOOGLE search and found this interesting data. 1" BORE and 1.125" BORE because of front DRUM or DISC brakes.

Gee, could someone in the past have used the wrong part? I didn't know they made ft Drum brakes for a 71 Chevy (My year, I looked up)

 

[John S]

I’ve got the second master cylinder you’re showing After 42 years as a mechanic that is the first thing I always think of when I have a problem when parts have been replaced. The first question I ask is what has changed and how and I usually go from there. It’s been seven years since I bought the car can’t remember exactly how well it stopped although it was not good. A coworker of mine before I retired had a 1970 impala and he had the same problem I believe he went to an aftermarket booster master cylinder and braking system and he tells me everything is fine

[cbolt]

So has the braking always been consistent, before you started working on it and replacing parts? Just curious if anything has changed since you started trying to fix your brakes. 

[John S]

12 minutes ago, cbolt said:

So has the braking always been consistent, before you started working on it and replacing parts? Just curious if anything has changed since you started trying to fix your brakes. 

I think the pedal feels harder with the new booster, even though it is working properly. The only thing that is different since the rebuild is the banjo bolts. The old ones had a 5/8 hex head while the ones I replaced were 7/16 head flange style bolt. Only difference between the two is the new one is 1/8 inch shorter Other than that, identical.

John S

[cbolt]

Im not seeing where you have inspected the pads and rotors, along with your drums, for glazing, unusual wear, etc. It almost sounds like something isnt quite lined up straight and you are only getting partial contact from your pads and/or shoes. Since from your first post we can eliminate sludge or air in the lines (did you also change the hard lines?), booster, master, proportioning valve, calipers, (I dont see wheel cylinders in your replace/repair list) master cylinder rod throw, then i would look at my shoes and pads to make sure they are making contact on the full surfaces. 

 

I experienced the same sensation when i installed my rear disk brakes. the axle housing flange was slightly bent causing terrible braking until i was able to diagnose and correct the problem. For whatever reason the drums didnt care the axle was a bit off but boy those disk brakes sure did. 

[wallaby]

I have been through the same process with mine. I changed everything ..with little improvement after all that work & expense.

My brakes are similar to what you describe; solid, but poor at stopping the car. I often wonder if I'm just spoiled by the brake performance of newer cars and expect the Monte to be similar.

I can share what DID work for me to make the brakes work better, but the differences aren't drastic. I never found "the culprit" that was clearly an obvious reason for poor brakes.

1. Brake shoes and pads. The Monte likes a soft lining for better bite and initial grab. The "lifetime", the metalics, and ceramic type of pad materials are far too hard for good braking. They may last forever, but don't offer up much friction. See if you can find a parts house that can supply you with organic linings...these are the old-school type that came with the car, and make your wheels dirty. This was the biggest difference I felt in all the parts changing I did.

2. This is a close 2nd to changing the pads & shoes. Have your drums & rotors resurfaced. I avoided this step for a long time because they looked fine...no groves or anything and they ran true.. and what could go wrong with those chunks of cast iron? Work hardening is an issue, and continuous use seems to just polish the surface making it harder for the linings to grab into. Don't bother to rough them up with a disc sander, you need to take them in and have them put on a lathe and have the hardened surface removed. This also made a difference I could feel.

Everything else I did was of little help. I finally went with a hydroboost system, but that came with a different list of issues. It seems to over-boost the brakes and makes them feel very touchy and gives very little pedal feel.

If I was still messing with the original booster system, I would be tempted to try the smaller master cylinder listed above for drum brake cars, just to get a bit more mechanical advantage. It might might result in a bit more pedal travel, but also delivers higher line pressure for the same amount of leg effort. It's a pretty low cost experiment.

[John S]

On 9/26/2020 at 7:28 AM, wallaby said:

I have been through the same process with mine. I changed everything ..with little improvement after all that work & expense.

My brakes are similar to what you describe; solid, but poor at stopping the car. I often wonder if I'm just spoiled by the brake performance of newer cars and expect the Monte to be similar.

I can share what DID work for me to make the brakes work better, but the differences aren't drastic. I never found "the culprit" that was clearly an obvious reason for poor brakes.

1. Brake shoes and pads. The Monte likes a soft lining for better bite and initial grab. The "lifetime", the metalics, and ceramic type of pad materials are far too hard for good braking. They may last forever, but don't offer up much friction. See if you can find a parts house that can supply you with organic linings...these are the old-school type that came with the car, and make your wheels dirty. This was the biggest difference I felt in all the parts changing I did.

2. This is a close 2nd to changing the pads & shoes. Have your drums & rotors resurfaced. I avoided this step for a long time because they looked fine...no groves or anything and they ran true.. and what could go wrong with those chunks of cast iron? Work hardening is an issue, and continuous use seems to just polish the surface making it harder for the linings to grab into. Don't bother to rough them up with a disc sander, you need to take them in and have them put on a lathe and have the hardened surface removed. This also made a difference I could feel.

Everything else I did was of little help. I finally went with a hydroboost system, but that came with a different list of issues. It seems to over-boost the brakes and makes them feel very touchy and gives very little pedal feel.

If I was still messing with the original booster system, I would be tempted to try the smaller master cylinder listed above for drum brake cars, just to get a bit more mechanical advantage. It might might result in a bit more pedal travel, but also delivers higher line pressure for the same amount of leg effort. It's a pretty low cost experiment.

Thanks for all the help. I have the softer organic brake pads. But I think the rotors definitely could be an issue. That’s an interesting thought about the master cylinder, I didn’t think about that. I’m not that fussy about having original rotors, I may try slotted  and drilled rotors. Also I may be expecting too much thanks again.

John S

[Traveler]

this may be too simplistic a solution but if you have the rotors turned make sure they put a non-directional finish on the rotors, this is just a fancy way of saying while still turning on the lathe ruff them up with a disk sander, and season the pads per manufacturer’s instruction, some require heat cycling others insist you break them in slowly